when you boot, see if you can get into your BIOS setup usually by pressing F1, del or esc (it varies between different motherboards)
and you should be able to enable it in the CPU options
if not, you may need a BIOS upgrade to use it
Hyperthreading is not always a good thing, and sometimes actually decreases gaming performance
it makes the OS believe that you have 2 CPU's, so it will send an instruction to the "second CPU" while the "first CPU" is already processing one. this way, the CPU can process more than one thing at a time
but a lot of applications, particularly games, are not optimised to take advantage of dual processors.