THe problem is fairly easy to understand, but far more complicated to fix.
The code you downloaded was poorly written and possibly for an older version of the .NET Framework, like 1.x.
You cannot touch any forms or their controls from a seperate thread other than the one that created the forms/controls. Fixing that may involve restructuring the application code and adding code to marshal these control manipulations to the correct thread.
] for the hows and whys.