Here's how to use Hexagon 2.5 for IMVU.
First, make the Object relative to the Scene as normal. For example a simple chair or couch. The mesh should initially be centred on the grid, underside flush with the ground plain, which can be adjusted later if necessary. It should also have a Material Diffuse colour, if not an image assigned. Once made, save the project locally then export using the "3D Studio" option. In the "Save As" dialogue make sure "Export scale factor" is set to "1.000" and "Unit of the file" is set to "None". Click "Save".
Next, in Autodesk's "FBX Converter" open the now available *.3DS file into an 'fbx convertor' instance (click "Add FBX Converter" button top-left). In the "Source files" section (left) make sure "Textures", "Materials", "Mesh", "Convert Units" and "Split Normals" are active, deselect everything else. In the "Destination files" section (right) ensure the resulting FBX is set to "Binary" mode then click "Convert" bottom-right.
With an FBX available, in IMVU, derive from the default chair furniture item. When the Editor loads click the "fbx import" tab then the "Import FBX" button. In the file dialogue, find and select the converted fbx file then click "Open" loading the data into the importer. Here, change "Mesh ID" to "0" to override the default, then for Hexagon created meshes, change "Apply scale:" to between "10" and "20" depending on the original size of the mesh in Hexagon. Click the "Import" button, then "Import Changes" in the "Import Status" pop-up. The file will then be parsed and converted, the import panel displaying blank when done. Finally click the "config" tab and select the derived items skeleton adding its seat to the mesh. Click "Apply Changes" to finish.