If you don't know, LED's have a positive side and a negative side, and will only light up when wired correctly. Most LED's have one longer lead to indicate the polarity, but not all. Here's a simple solution to find the correct side BEFORE installing it into your project.
Step 1: Equiptment List
2: Wire strippers/cutters. C'mon, you don't have a pair these in your back pocket already?
Step 2: What to Do
3. MOST IMPORTANT! Reward yourself with a cold beer. (if under 21, milk will substitute)
Step 3: How to Use
2. Install your LED into your project correctly the first time.
Step 4: Other Uses
Now, after I finish this case left over from step 2, number 3, I'll check the comments section and answer any questions you might have.