alt_signaltap_run can't work correctly.

You are right about genpath, I had already tried only

addpath('C:\altera\91\quartus\bin')

but it did not work, so in my desperation I added all the subfolders (if any) to Matlab's path.

I guess I will export the signaltap data to a text file and import it back to matlab, seems a good option for debugging, although not very good for a practical implementation.

My guess is that the Altera mex files were compiled as a 32 bit application, and they cannot be called directly in a 64-bit OS. Could you confirm if your XP is 32-bit?