Please help me XILINX 9.1+Modelsim se plus 6.0d

Dear guys,
I done a simple schematic by using a xilinx program and when I use a modelsim there is an error but I don't know why this error.

# Reading C:/Modeltech_6.0d/tcl/vsim/pref.tcl
# // ModelSim SE 6.0d Apr 25 2005
# //
# // Copyright Mentor Graphics Corporation 2005
# // All Rights Reserved.
# //
# // THIS WORK CONTAINS TRADE SECRET AND
# // PROPRIETARY INFORMATION WHICH IS THE PROPERTY
# // OF MENTOR GRAPHICS CORPORATION OR ITS LICENSORS
# // AND IS SUBJECT TO LICENSE TERMS.
# //
# do {test2.fdo}
# ** Warning: (vlib-34) Library already exists at "work".
# Model Technology ModelSim SE vcom 6.0d Compiler 2005.04 Apr 25 2005
# -- Loading package standard
# -- Loading package std_logic_1164
# -- Loading package numeric_std
# ** Error: (vcom-19) Failed to access library 'unisim' at "unisim".
# No such file or directory. (errno = ENOENT)
# ** Error: C:/Modeltech_6.0d/win32/vcom failed.
# Error in macro ./test2.fdo line 5
# C:/Modeltech_6.0d/win32/vcom failed.
# while executing
# "vcom -explicit -93 "testsh.vhf""


Please help me.
Thanks in advance.

Check your environment variables:

User Variables

VSIMBIN C:\MentorGraphics //Should point to the installation directory

System Variables

Path C:\MentorGraphics\Licensing //Contains this as well as many refs


These are available under ControlPanel/System/Advanced/Environment Variables