Before we begin, download and install the latest version of Unity3D (http://unity3d.com/get-unity). You don't need to install all the platforms and examples – just the Editor is necessary, but if you'd like to get into game development, feel free to add the documentation, example scenes, standard assets, etc. Visual Studio Community is also a pretty neat C# IDE that you can install from here, too.
Once installed (or during), navigate to VIRTUAnimator's Steam directory (Default: c:/Program Files/Steam/steamapps/common/VIRTUAnimator), and locate the VIRTUAnimator_SDK folder. Inside is a .ZIP file. Go ahead and extract the files, to a location of your choosing (keep note of where; we'll need that in a moment).
Once everything is installed/extracted, go ahead and open Unity – in the newly-opened dialogue, locate the Open button, navigate to where you extracted the SDK, and open it.
As a general rule, DO NOT delete or modify any existing files. Things will break, and you'll need to extract the SDK all over again.
The main folders that'll be of interest to you are 'Content' (ideally where you'll put your main assets), and 'Your Items' (where your working assets will be).
Firstly, bring in all the relevant items for your object (obj/fbx, textures, etc) and put them in the Content folder. Next, locate the Workshop menu button near the top of the Unity3D editor (the same line as File, Edit, etc.), click it, and select 'Create New Workshop Item'. Pick a descriptive-yet-short name for your item, pick whether it's an Item or a Scene, then hit Create. This will now create a new folder with the chosen name in the 'Your Items' folder.
Drag the Item object (with the blue cube icon) into the Hierarchy (or the Scene). Drag the object from the Content folder into the scene as well. Once both objects are in the Hierarchy, drag your object onto the Item object to make it a child of the Item object. Position the object (the child object) so that the red mesh of the Item is in a convenient location. Select the Item object in the Hierarchy, and hit the Apply button at the top of the Inspector to apply the changes to the Prefab.
In the Game view, set the resolution (the drop-down next to the drop-down labelled 'Display 1') to 512x512 (create a custom resolution if it's not there by default). Once you have the item all set up, position the camera in the scene to get a good view. Hit the Print Screen button on your keyboard, open up your image editor of choice (I prefer Paint.NET (http://www.getpaint.net), and past the image in. Crop it so that only the 512x512 Game view is visible (the image should also be exactly 512x512). Save this image to the Item's folder as icon.png (overwrite the pre-existing icon.png there).
In the itemmeta file in the same folder, set the Title, the Description, the Preview Image (this should already be set to the image you saved just before, but set it if not already), the Item (should be the Item object by default), and any Change Notes at the bottom (usually only if you're updating the asset).
If the Item ID is set to 0, it will generate a new ID when you export the file. If you're updating an item, set it's Item ID to the Workshop ID of the item, or the ID of the Local folder you've exported to.
When everything is all set up, select the itemmeta file, and hit Check for Problems at the bottom. Any problems will be displayed above. If no errors are shown, you'll now be able to Export Local, or Create & Upload (to submit the item to the Steam Workshop).
After uploading to the Workshop, you'll be taken to the item's page. If you exported local, you can find the files in the same directory as the SDK. Be sure to copy the Item's containing folder (a random number for a name) when copying it to your Steam install of VIRTUAnimator.
For rigged characters, follow most of the steps for simple objects. Some things to note: your rigged model MUST be recognized and correctly setup for Mecanim. To see if your model is correctly setup, select the original object (in the Content folder), and click the Rig button near the top of the Inspector. Ensure the Animation Type is set to Humanoid, and that the Avatar Definition is set to Create From This Model. After changing these, hit Apply, and wait for it to finish. If successful, there should be a Tick next to the Configure… button. If there isn't, click Configure, and manually set it up (most correctly setup characters should work automatically, though).
Once setup, and the character object is a child of the Item and the prefab has been applied, select the Item object in its folder, and locate the ObjectData component attached. Change the Special Features option to Biped IK. If you'd like to add pins to the character's hands (for grabbing props/weapons and such), navigate through it's hierarchy until you get to its Hand or Wrist transforms. Drag the Pin prefab (in the Workshop folder) into the Hand/Wrist transform to make it a child. Position it appropriately to center the hand. Apply the prefab again.
Follow the same export steps as above to export/update your character.