Hi Audrey,
It really depends on which companies you sign up with. Some will email jobs available. Other you have to check out their sites to find what jobs are available. When I've signed up with a company and see there are no jobs available, I will try to search outside my area or state to see if they even have any around. It could be also that you are just checking at the wrong times of the month.
Keep your chin up. Look at the list of companies posted on this site and register with more companies. The more you're signed up with, the better chance you'll have of jobs coming your way.
Where do you live? May be fellow shoppers can direct you to companies they see are in your area.