The H.264 encoder should, by default, utilize as many threads as you have logical cores (via OpenMP). Can you confirm that your system "sees" multiple cores? (Note that an HT physical core should appear as two logical cores.)
On a Windows system the easiest way to confirm that your system sees multiple logical cores is to bring up the Task Manager and look at the Performance tab. In the CPU Usage History area of the dialog box you'll have one graph per logical core.
Are you using the dynamic linking model? Which version of the IPP library are you using?
Regards,
Paul