In some games, open-limping with low pocket pairs in early position is more profitable than folding. Since it is not clear that raising with a low pocket pair in early position is profitable, it is correct in some games to open-limp. For this to be a reasonable play, you want a good chance that no one will substantial raise after you limp. In some games, it is not good to open-limp with a low pair, and folding may be the best option in early position.
Usually do not open-limp in late position. Usually, either raising or folding will be better than open-limping in late position.
Overlimping is much more commonly correct than open-limping. Don't forget the alternative of raising. Just as you open-raise 10 bb comfortably, you may shove after limpers for about 10 + 3 x #limpers big blinds without it being an overbet. However, sometimes it is right to overlimp, and hope to hit the flop hard and get paid off. People play very badly in the first level, so look for excuses to play hands while people are giving chips away.
If you limp in with a low pocket pair, and miss, this does not mean you should give up on the pot. Look for chances to take pots no one wants. It does not matter much whether you have the best hand. Chances to take the pot occur much more often when you have position, but if people check the flop behind you, consider betting the turn.
Medium pairs 66-99 are significantly stronger than low pocket pairs 22-55. They more often win unimproved, they make more straights, and they get counterfeited less often.