From Linux on Power wiki
Also known as DLPAR, Dynamic Logical PARtioning allows you to add/remove/move resources from partitions while they are running. Resources include cpus and io slots. It's very handy.
There is free, as in free beer, software at IBM's site for SLES and RHEL. Install these rpms on your partition you wish to be able to do DLPAR on. Make sure that this partition has a network connection to the HMC. Suddenly your HMC should be able to do DLPAR with that partition.
Dynamic support for memory is not yet available under Linux. If you wish to follow the progress of dynamic memory under Linux there is a mailing list complete with archives. Any changes to memory assignement will be visible after shutting down and reactivating the partition. (init 0, not init 6).
There is a pretty GUI on the HMC that should be pretty self explanitory.
see http://www-128.ibm.com/developerworks/linux/library/l-pow-dynamic/ Dynamic logical partitioning for Linux on POWER
Setting up and using dynamic LPAR