AFAIK, you have to rebuild the kernel if PAE support isn't there and you want it. Rebuilding the kernel is generally easy if you start from a working config file, but it takes a while (10's of minutes). For example, if you're using Debian Sarge and a 2.6.x kernel, see this link: http://www.howtoforge.com/forums/showthread.php?t=21
but of course change the kernel version to match the one you have. Copy your /boot/config<your-version> to /usr/src/linux-<your-version>/.config. Assuming you do this and the config file is in the right place, and that you need no other changes, when you run "make menuconfig" all you need to do is turn on the appropriate option for PAE (for me this was "Processor type and features ---> High Memory Support ---> 64GB" but you may want only 4GB, and/or YMMV depending on the kernel version you've got). Good luck.