To deallocate a CPU you need to access the Service Processor Menu (or ASMI on System p5 servers), not the SMS menu. Accessing the SPM (ASMI) is pretty simple:
(a) You have a serial console attached to S1: halt the OS, wait for the service processor to reset and press enter on the console and you're in. Type your ASMI username and password (admin/admin by default) and you'll have the menu.
(b) [p5 only] You have your notebook connected to the HMC1 ethernet port (of course, this only applies to full partition/no-LPAR systems): open a web browser and connect to
https://192.168.2.147, type your ASMI username and password (see above) and you'll have the full menu.
(c) [p5 only] I believe you can access the ASMI using the HMC2 port even if the HMC1 port is connected to an active HMC. Address is
https://192.168.3.147, I think.
(d) If all else fails, read the HW manual for your system
