You already signed the letter, so technically the company doesn't have to pay you your verbal bonus. Definitely ask about it to HR because you don't lose anything by doing so, but don't expect them to correct the mistake. In their eyes, they just got an engineer on the cheap, which benefits them immensely.

If they don't correct it, look at your compensation and decide if it's still worth working there. If not, polish up that resume! Remember, business is business. If you're not being paid fairly, you have every right to leave for something better, and don't feel bad about it.