[R] How can I use muliple cores of CPU in Windows or OS X?

[Agony]

Dear all,

I have about 50 pages of R codes and ran it in both OS X and Windows.
It takes at least haft a day to have the results. The running time is not very different in both Systems.
I found that R does not use all cores of CPU by default.
Can anybody help me to use all cores of CPU in my programming from the beginning or through programming in both OS X and Windows?

Many thanks your attention and help in advance.

Best,
Amir