If you are having trouble understanding levers check out the following sites for extra practice:
http://edtech2.boisestate.edu/scangaj/506/simple_machines_1_1.html
http://edtech2.boisestate.edu/scangaj/506/simple_machines_1_2.html
http://edtech2.boisestate.edu/scangaj/506/simple_machines_1_3.html
http://edtech2.boisestate.edu/scangaj/506/simple_machines_2_1.html
This site is a great practice for figuring out different types of levers.
http://www.edheads.org/activities/simple-machines/index.htm