FreeBSD has a built in boot manager which can be installed at the time of system installation, to allow the user to select from multiple operating systems installed on various hard drives or partitions. It's pretty basic, just presenting the user with a (default ten second) chance to press F1 for selection 1, etc.
If you did not install the boot manager when you installed the OS, and you want it now, you can either use the boot0cfg utility to install it from the command line, or you can use the sysinstall method. If you did install the boot manager, but now you want to get rid of it, you'll need to use the sysinstall method no matter what.
In order to change boot managers with sysinstall, you'll need the boot CD for the OS - sysinstall can't write to the MBR of the active drive! Boot from the install CD, and when you get to the sysinstall main menu, select Index --> Fdisk. Just hit "W" here for "Write Changes" - and don't panic, you haven't made any changes here, so we aren't clobbering your drive. After confirming that yes, you do wish to write your changes, you are then given the option to select the freebsd boot manager or no boot manager at all. Pick whichever you like, the MBR will be re-written, and you're done!