You can do one of two things..... Pay the 11.95 for the domain registration now and when you end your hosting trial and begin paying for hosting make that domain your free domain which means 11.95 will be subtracted what you pay then. The second is of course just wait until the end of the free hosting trial.
The reason you have to pay up front for the domain registration is because it's not a trial. If you register the domain it is yours to use for a year. You can host it at dreamhost or point it elsewhere for hosting either way it's yours for 1 year.
here is a wiki article on the subject: http://wiki.dreamhost.com/Free_Trial_and_Domain_Registration_Deposit