When I try to open a custom prop in the Model Viewer I get an error saying: "Error loading model" and they don't work in Hammer at all. I couldn't find barely any examples of people trying to get custom props into Portal 2. Has anyone here successfully done this? Here's my workflow, maybe someone could tell me what I'm doing wrong.

I'm using the smd plugin for maya: http://developer.valvesoftware.com/wiki/Modifying_a_Source_Model_in_Maya

1. Create a cube in Maya.
2. Save in location: C:\Users\Home\Documents\maya\projects\default\scenes - as "cube1"
3. Select model in Maya and go File/Export SMD. This creates a new SMD file: C:\Users\Home\Documents\maya\projects\default\smd - named "cube1.smd".
4. Duplicate this file and name it "cube1_collision.smd" and put it in the same folder and put a copy of idle.smd in the same folder.
5. Open up a new notepad and paste in:

$modelname "cube1\cube1.mdl"
$cdmaterials "models\cube1/"

$scale 1.0
$surfaceprop "Metal"

$body "Body" "cube1.smd"

$sequence "idle" "cube1.smd" fps 30

$collisionmodel "cube1_collision.smd"
{
$mass 5
}

1. In notepad: File/Save As
2. Save in: C:\Users\Home\Documents\maya\projects\default\smd
3. Save as type: All Files
4. File Name: cube1.qc

1. Open GUIStudioMDL
2. Go to: Config/Set EP1 Tools Path: C:\Program Files (x86)\Steam\steamapps\common\portal 2\bin
3. Go to: Config/Set Orange Box Tools Path: C:\Program Files (x86)\Steam\steamapps\common\portal 2\bin
4. Target Mod or "GameInfo" Path/Add: C:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\gameinfo
5. Go to: File/Load QC File: C:\Users\Home\Documents\maya\projects\default\smd\ cube1.qc
6. Go to: Actions/Compile - This creates a new folder: C:\Program Files (x86)\Steam\steamapps\common\portal 2\portal2\models\cube1\
7. In that folder are 3 new files: cube1.dx90 - cube1.mdl - cube1.vvd

1. Open Steam/Library/TOOLS/Portal 2 Authoring Tools
2. In Portal 2 Authoring Tools open Model Viewer
3. In Model Viewer: File/Load Model
4. Open: [ROOT]\models\cube1\cube1.mdl

When I do this I get an error message: "Error loading model". Does anyone know what I'm doing wrong?

A broken model can have a lot of reasons. Problems in the smd, problems in the .qc or problems while compiling.
For me your .qc file looks good.
Most times if there is a problem the compiling process gets aborted. If it's ok you could upload the smd for me and I'll try to find the problem for you.

Here's the .smd file:
http://forums.thinking.withportals.com/downloads.php?view=detail&df_id=1128

If the .smd exporter is the problem then I can try blender or 3ds Max instead.

oops, that would make more sense I suppose. Also; I tried the Wall Worm Model Tools and I get the same error in the model viewer, dang it.