Packaged models should be viewable with the Model Viewer but Alien Swarm‘s aren’t. That’s because the Model Viewer requires a proper folder/directory structure even if said folders don’t actually contain any models. Otherwise you’ll get a bunch errors.
To view Alien Swarm models, then, you’ll need to create a whole bunch of empty folders in “C:\Program Files (x86)\Steam\steamapps\common\alien swarm\platform\Models”. That’s a bit of a hassle to do manually, though, so I used TreeCopy to make a copy of the folder/directory structure (but no actual files) created after extracting the “pak01_dir.vpk” file with Gibbed’s VPK Extractor. Like so:
The files extracted from the VPK weigh in at over 500mb, so if (like me) you just want to browse through the models and not actually manipulate them, this is a much nicer solution. To make life easier I’ve created a ZIP file copy of the relevant folder/directory structure which you can download here. Just extract it over your existing “Models” folder and you should be good to go.