There are three ways to display imported mechs:
1. Toolbar button "Play Game" should show the mechs walking in the air.
Use the asdw and mn keys to navigate.
2. Toolbar button "Mech Bay" should show a list of imported mechs
and one mech walking. Click the mechs in the list to see them.
3. Open the .OBJ files in the mechs subdirectory with any 3D viewer.
If none of this worked, check if any of the mech folders contains .OBJ files.
For example, Mad Dog should be present. Otherwise, importing failed.