suethechmist has not logged into the Forums since 2007 so you will probably not get a reply.
To get good answers you need to ask good questions. "...but it doesn't seem to work very much." is not very infromative.
1. What signal are you using as the input to the PID VI?
2. What are you doing with the output of the PID?
3. How are you measuring the laser frequency/wavelength?
4. How are you controlling the laser frequency/wavelength?
5. What role does the lock-in play? Are you using a hardware lock-in or are you implementing it in LV?
6. Tell us about the interfaces between the LV program and the laser equipment: What DAQ devices do you use? How do you communicate with external instruments - GPIB, RS-232, TCP/IP, other?
7. What quantitative indications do you have for the evaluation of how ell it works?
Lynn