For Importing GTA .dff you need to use 3DS Max and install the addon "Kam's Max Scripts"
http://www.gtagarage.com/mods/show.php?id=9172Then use that script to import your .dff model,
1. In 3ds max, click the hammer icon in the right panel, after that click IFP IO (see attachment 1)
2. After that a window will pop out, just click the import dff and search your .dff model (see attachment 2)
3. after that your model will load in the 3ds max
4. then after that just export it out in .obj in 3DS Max
If you mean rotating mesh in blender, then
In Object mode, just click RMB (right mouse button) the mesh, and press R Key and rotate it after that press the Space Key or Enter Key to confirm the rotation