Version 5.5b of the ModelSim-Altera software upgrades certain device drivers, which causes incompatibility. These new device drivers disable the T-guard licensing mechanism. Re-installing the previous version of the ModelSim tool does not resolve the problem.
To enable the software with your T-guard, you must download the mgls.dll file from Altera's web site (http://www.altera.com/products/software/download/dnl-download_dl.html).
Once you download the file, copy it into the /win32aloem directory. This allows you to enable the software with your T-guard license file.
This problem was fixed in the ModelSim software version 5.5e.