Your poser serial number file is kept in a different location on your computer depending on what operating system you are on? Are you using a mac or windows?

For windows that is under C:Windows:ProgramData:Poser:11 and in there you should see 3 files. Poser.dta and Poser Activation.dta and PoserReg.dta If you have your full serial number you can safely delete those files and then when Poser opens again you can put in the serial/registration key and register as usual. It may be a case of not putting a space in the front of the serial number or after.