Open the object editor (F6).
The "Unit" tab should be open by default, if not: open the unit tab.
Now go to Human -> Melee -> Units and select the Peasant.
To the left, you see a set of values, select "Art - Model file" and select your imported model.
If the model does not work, then the texture path is wrong.
Open the import manager again, open the readme-file of the model (every file has one) and check the path names.
If these don't match, then change the path name of the textures to what the readme-file says.