can't find where to access a beginner puzzle
After you have downloaded and installed the Foldit game client, run it and log in with your user name and password with the "Play Online" button.
The beginner puzzles will be under the "Science Puzzles" section. You may need to click the checkbox before the "Show beginner puzzles" so that the beginner puzzles will show up the list. Scroll to find the one you want, and click "play".
See also http://fold.it/portal/node/988864 for more information for beginners, and be sure to check out the tutorials for more information on how to play the game. If you need help using Foldit in general (rather than comments and questions about a puzzle in particular), try posting in the forum. Other Foldit users hang out there and can give you advice.