For a tank right click in your 2d view and go to static --->Vehicle and find the 1 you want from there.
As for loading in prefabs im assuming the file you got is a .pfb file as they are the prefab files. To load this into the map simply create a folder in your MOHTools directory titled prefab, then go to edit --->preferences and at the bottom is a spot where you can tell Radiant to pint at the prefab folder. Then all you simply need to do is go to file ---> load prefab and your prefab shall appear in a list, select the prefab you want and load into map.
ps, forgot to mention this but you have to put your .pfb file in the newly made prefab folder too (obviously)
