Summarize the following:
It's usually in the upper-right part of the keyboard.  It's usually on or near the J, K, or L keys. A heart (♥) symbol will appear where you placed the cursor. Even if the keys are unlabeled, the keypad will still work when Num Lock is on.

Summary:
Click on the location where you want to insert the heart. Press NumLock. Press Alt. Press the small 3 on the virtual keypad.