Last I checked (2 years ago), you can compile a call to the Neural Network Toolbox "sim" function. You cant compile any type of training functions (train, adapt, etc.), but simulating a pre-trained network shouldnt be an issue.
Looking at your code snippet above, if I had to guess, I'd say you're running into a problem with your call to "load". The compiled function may not be running in the active directory where "SIMdata.mat" is located. Try using a full path just in case that is that is the issue? You cna always revise later once you have a solution working.
Also, I would try using the form of load that captures an output:
out = load('SIMdata.mat')
T2 = sim(out.net,[W1;T1;RH1;S1]);
Ive run into issues in the past with this as the compiler doesnt know exactly which version of "sim" to include in the package without being able to sope the run time type of "net".
Hope this helps.