I receive an error message when trying to carry out a Pro Search, Warning - you are running a search that uses Professional Sources and you have insufficient pro credits.

This search will be charged at 10 credits

Although your account states 0 Pro Credits, you are able to still carry out Pro Searches as the system will use 10 Standard Credits to carry it out.
ie. Having 46 Pro Credits equates to 460 Standard Credits, either will work for you.