what does "middle click" to lock mean?

[twokidz2]

I have a left and a right click on my mouse, and am stumped on puzzle 4-2. I ran right through the first puzzles in less than 20 minutes, but no matter what I do, I can't get the out of place sheet to line up with the other sheet. Every time I try to pull it into place, it stays locked in position relative to the sheet it should be parallel with and misshapes the rest of the protien. What am I missing here?

Thanks

[gurch]

If you don't have a middle mouse button, Shift + left click should have the same effect. (If you have a scroll wheel rather than a middle mouse button, you can also press down on that to "click" it).