In this puzzle, the tweak tool is used to "flip" a sheet. This turns the sheet over and moves it over a bit. In this case, flipping the sheet will hide the two hydrophobics in the core of the protein.
Tip: the tweak tool is found on the pie menu in Pull Mode. Hover over the protein and right-click (control-click) or shift-click to open the pie menu. If you'd prefer to use middle click, open General Options from the main menu or using control + t, and check "switch right/middle click".
This puzzle can be a little touchy, so you may need to reset and try again a time or two.
First, right-click anywhere on the second sheet and select "Tweak" from the wheel menu. The tweak tool shows purple arrows next to the sheet. Click on the right side of one of the arrows to flip the sheet. The sheet turns over, so the exposed sidechains are now facing away from you. The sheet also moves toward the lower right, shifting over so it stays bonded to the two adjacent sheets. Be sure to stop the Tweak before trying anything else.
Flipping the sheet is the easy part. If you wiggle at this point, the protein will fly apart. If you shake, then wiggle, the protein will hold together, but you may not be able to get the required score. (Even if you repeat shake and wiggle.) You need a "local" strategy, that works on the worst parts of the protein first.
When you flip the sheet, notice that the section of loop on the right between the top two sheets turns red. That means this section has problems. Right-click (or control-click) on one of the loop segments and select "Shake" from the wheel menu. Let it shake for a bit, then stop and select "Wiggle" from the wheel menu. Using the right-click wheel menu means shake and wiggle affect only the loop (or other secondary structure) that you clicked on.
After shaking and wiggling the bad loop, try shaking and wiggling the entire protein. This should allow you to finish the puzzle.
There are other techniques to keep the protein from flying apart after flipping the sheet. These methods are "global", meaning that you shake and wiggle the whole protein at once instead of working on a small section.
One alternative is to freeze all four sheets by shift double-clicking on each one. (You can also select "Freeze Structure" from the right-click wheel menu.) After freezing the sheets, start to wiggle. The protein will still pull apart, but it should start to come together again. When the sheets are bonded again, stop the wiggle and shake. Stop the shake, and click on "Unfreeze Protein". Then wiggle again. This should be enough to complete the puzzle.
As usual, you can shake and wiggle a few more times to improve your score.
You can also draw bands between the sheets to hold them together, instead of or in addition to freezing them.
Technical stuff: yes, Foldit has bugs. The Computer Science kind, not the Biology kind. This puzzle seems to be a problem from time to time.
Foldit player LennStar pointed out that this intro puzzle had problems way back in 2009. Sometimes you just can't get the required 8800 points. Try resetting and using a different technique if this happens to you.