Dotfuscator Professional edition integrates with VS 2005/2008/MSBuild, so obfuscation will happen automatically when building a solution containing a Dofuscator Project via the Make VS 2005/2008 actions.
Alternatively, you could invoke their command-line app from a Run Program action. The Community Edition command-line flags are documented at
http://msdn2.microsoft.com/en-us/lib...38(VS.80).aspx (VS 2005) and
http://msdn.microsoft.com/en-us/library/ms227238.aspx (VS 2008). For other editions, consult the product documentation.