When pool mining, how do I get paid?
You need to get a receiving address from the client (or from an e-wallet if you prefer) and copy and paste it into the address field against your account in BTCGuild. 

Bear in mind that if you're using the Bitcoin-QT client there is a very long synchronisation process to wait for, though you may be able to get a receiving address before it completes (but you won't see any received bitcoins in your wallet until it's synchronised). Depending on your hardware and bandwidth, synchronisation may take several days to complete.

While there's no receiving address against your BTCGuild account, you'll just be accumulating bitcoins against your BTCGuild balance, so you aren't losing anything in the meantime.

Bear in mind that the "difficulty" level of mining for the network as a whole has risen to a very high level due to the arrival of highly efficient dedicated ASIC bitcoin mining hardware. This means mining on graphics cards is likely no longer profitable (depending on purchase and electricity costs) and CPU mining hasn't been profitable for a long time.