Im using this...it says to use the VCMod Manager and open the .vcm file, but all the files are .dtt or other extensions from this list:
http://www.thegtaplace.com/downloads/category.php?id=9
What do I do with the .vcm file?
Well first you need to extract it from the zip/rar file you downloaded, you will need to extract it to the 'vcm' directory where you installed VCMM. By default this is C:\Program Files\vcmm\mods\vcm but if you installed it elsewhere then you will have to find that directory yourself.
Now how do I install it?
This is the easy part. Just start up VCMM and find the mod you just downloaded. It will be listed as either a 'Car' or 'General' mod. If it's a general mod then you can just double click it and it will install (or click it once and press install).
If it's a car then when you click to install it a list of vehicles will appear, you simply select which vehicle you want it installed as and your done.