SeeLevel does pretty well paying folks to the best of my knowledge. Once you have money in PayPal, it will remain there (it's your money and your account) until you take it out.
I always check my bases first when I have an issue. So, I'll ask: Did you verify that SeeLevel has the correct PayPal address for you? I know it sounds silly, but there have been shoppers who gave MSC's the wrong email address for their PayPal. It's easy to check your profile to be certain.
Once you have confirmed that the they have your correct account information, I'd contact PayPal. Tell them the date SeeLevel says they paid you and see if they can see something.
Hard work builds character and homework is good for your soul.