I also used the Load Multisample dialogue to force Dim-Pro to load the REX file, but nothing, nada, zippo. No error, but no load either.
I'm on Mac Snow OS X 1.6.6, Logic Pro 9.1.3, Dim Pro 1.5.
Behaviour is the same with the microhost. For the life of me, I can't find the solution.
(I can import the REX files into Logic directly, if that helps... but they should work within Dim Pro as expected, right?)