Did you install the prereqs for Mathcad? You need the .NET framework. The version depends on the Mathcad version, use the version that is on the distribution CD (I think that MC13 needs .NET version 2). Also the XML support. The readme on the CD should tell you all the prereqs required -- there may be a data access module as well (although I think that is actually more or less optional, and affects only some particular I/O functions).