try a 'locate startx'

if thats not there no x11

I DONT recomment compiling it from the ports tree as that is slow

so i would burn all of the packages for x11 onto a cd and pk_add them
otherwise download ports and untar it
cd /usr/ports/x11

make install

PREPARE TO WAIT FOR DAYS
well i dunno about days anymore... well just longer than i like to wait