http://bugs.winehq.org/show_bug.cgi?id=31185
--- Comment #2 from Hans Leidekker hans@meelstraat.net 2012-07-11 06:49:15 CDT --- (In reply to comment #1)
I can't quite reproduce that problem, but Mathcad 15 requires .net 3.5sp1 to install. It doesn't put up that error unless you do 'winetricks dotnet20' before installing. Sadly, 'winetricks dotnet35sp1' doesn't work yet.
.NET 3.5 SP1 should install on Wine, with some workarounds. I remember installing .NET 2.0, .NET 2.0 SP2, .NET 3.0 and XPSEP as prerequisites, in that order.