Download the compiler tools from the Mentor Graphics website. Note, Code Sourcery is now available only to registered users and no longer available free of charge.
If using v2.40 or later: Edit the Common/build.mak file to have the correct ARM_TOOLS_PATH and ARM_TOOLS_PREFIX variables.
If using v2.36 or earlier: Add the bin directory of the cross-compiler tools to the system path environment variable.