This is a problem in the evaluation version of the Modeler. We will fix this as soon as possible. We apologize for the inconvenience.
If you are adventurous, there is a way to get your Modeler running on a 64-bit OS. You have to force it to run in 32-bit mode:
corflags /32BIT+ /Force Modeler.exe
Thank, that fixed the error.
By the way, the full commands are:
cd "C:\Program Files\Microsoft.NET\SDK\v2.0 64bit\Bin"
CorFlags /32BIT+ /Force "C:\Program Files (x86)\Mendix\2.4.3.1 Evaluation\Modeler.exe"
Just copy&paste!