tmoran1,
I am not sure what error you are seeing, and will have trouble diagnosing it without seeing your VI. However, I have extended my example such that it should be compatible with your DAQPad for your input voltage source. It uses software timed reads to acquire a voltage, then checks it against the limit. This is a fairly simple method which should get you started. The rate at which this type of read will execute is dependent on your system, and your system's load. If it does not acquire data fast enough (ie.. you are missing transitions above your threshold) then you may want to consider using a hardware timed approach.
Hope this helps,
Dan