Floating point is very, very, very, very rarely (if ever) used inside the kernel itself.
Userspace apps use floating point a lot, and those instructions are enabled for userspace apps. You'll see these enabled for applications that actually make use of them. Watch the output from compiling ports, you'll see them enabled in there.
MMX is like a horse, in that it is evil and it eventually tries to kill you
If you set [red]CPUTYPE?= [/red] to one of " opteron, athlon64, nocona, prescott, core2" in /etc/make.conf (depending of course on what you have) the system should automatically use those SIMD extensions when compiling (in places where it is appropriate (which won't be within the kernel)).