Jorge,
I know there were some problems with the LabVIEW 5.1 Application Builder that were fixed in the LabVIEW 5.1.1 Updater. I recommend you download that update to see if this fixes the problem. Also, double-check to make sure you have typed the file paths correctly. If you continue to have problems after installing the 5.1.1 updater, please let me know, and please attach your .bld file so I can examine it for any problems.
Thanks for your patience on this issue, and have a pleasant day.
Sincerely,
Darren Nattinger
Applications Engineer
National Instruments