Steps:
1. Perform the Shortcut: Press the F2 Key.
2. If you performed this easy step it should say something like "Saved screenshot as _insertcurrentdatehere_"
3. Now navigate to your .minecraft folder. If you know how to find it then skip to Step 6.
4. Now go to your RUN program. (Find in "All programs")
5. Type in "%APPDATA%" without the quote.
6. Open your .minecraft folder and look for the screenshots folder.
7. Your Screenshot should be in there