Dear Dale,
That message means that the time step of your weather file (60 minutes) does not match the time step of your load data (15-minutes).
Ideally you should use weather data that matches your load data, because it is likely that the load is affected by the weather data. For example, air conditioning loads increase with solar irradiance and temperature.
If your load data is from a particular year, let's say 2013, then you could download a single-year weather file from the NSRDB for 2013 instead of using the TMY file.
We have developed an LK script to convert your hourly weather file to a 15-minute weather file. That will at least allow you to run a simulation and get some results. To use the script, download it from the Samples page (it should be Item #2):
https://sam.nrel.gov/samples
To run the script, in SAM go to File, Open script, and then click Run at the top of the script editor.
Best regards,
Paul.