It is very difficult to install Modelsim on 64-bit Win7.
I'm not sure if you're talking about the webPack or not, but the
workaround for the WebPack is the following:
1. Downolad the .tar file
2. Unrar it
3. goto the extracted folder\webpack\lib\nt
4. copy all the files
5. goto extracted folder\webpack\bin\nt
6. paste the files
7. add an environment variable called Xilinx and put in the C:\Xilinx
8. Create the folder Xilinx in C
9. Install
Should work mostly fine after that.