Since I'm not familiar with BootMagic (or
FreeBSD for that matter) I would not claim to
"know" exactly what you should do however,
I will direct you to the following URLs:

http://www.ccp14.ac.uk/solution/bsdunix/

http://unix.about.com/library/weekly/aa041601c.htm

...from what I understand, FreeBSD also has a
boot manager similar to Linux's LILO. I think
these sites can give you much more accurate
information than I can