By default your Mac should accept commands when the firmware initializes, which allows you to send boot arguments to the OS X kernel, and perform other tasks such as resetting the system's PRAM. In order to do this, you hold down various key combinations at bootup; however, in some instances the computer may not recognize these key combinations at boot.
There are a couple of reasons why the system may not allow boot arguments to be input from the keyboard at system startup:
Firmware password set
The most prevalent reason for this is if the system has a firmware password set. This can be done using an OS X boot DVD which contains a firmware password utility. If the password is set, then the only command the only key command the system will allow at boot is the options key, which will bring up the boot menu. To disable the firmware password, boot from the OS X installation DVD and select your language. Then from the utilities menu choose the Firmware Password Utility, and follow the instructions to remove the password.
Try a different USB port
It is possible that some USB ports may not be fully functioning at boot. A recent Apple Discussion poster found that this was the case with his system, and switching the keyboard to another USB port fixed the problem. Additionally, be sure you connect your keyboard directly to the system instead of going through a USB hub. Doing this will ensure there is no interference between the keyboard and the system, especially during boot when various USB devices (including hubs) may be initialized at different stages of boot.