How to Add the Xilinx Library to Modelsim?
First of all it would be good to know which version of Modelsim/ISE you are using. In general the Xilinx simulation libraries have to be compiled. Only ISim ha pre-compiled binaries.

The compilation can be done with the command-line tool compxlib that is supplied with ISE. The usage is described in Command Line Tools User Guide (v14.4) - the link points to the most current version of this file. Chapter 25 (p.321-335) cover all options for this tool. The most important options are:


-s sets the target simulator should be mti_{se|pe|de}
-arch for the Xilinx FPGA architecture, e.g. spartan6
-l to specify the language so you have to use verilog
-dir to set the output directory of the compiled libraries (if you have write permissions to the Xilinx ISE directory you can omitt this and all files will be put in the ISE directory according to p.325)
-p is used to set the path to your simulator. Not neccessary if the simulator is on the $PATH environmental variable.


I have tested this and successfully compiled all libraries including unisims_ver. As I only work with VHDL I do not know the exact usage of the libraries with Modelsim. After a short search I found the Modelsim User Manual that describes the usage of libraries on the pages 277 till 283. Maybe some experienced Verilog developer can explain this better.

Also I do not know if The Altera Modelsim version can be used to simulate Xilinx hardware as I only work with plain Modelsim.

This is my first post on StackExchange. I hope my answer is able to help you and sticks to the rules.