linux compatibility on 64 bit FreeBSD 5.4 and 6.0

[Philip Sydney Lavers]

Gentlemen,

I have spent several days reading all the documentation I can find but no reference to my problem.

linux compatibility:
# kldload linux# 
   OR
linux_enable="YES"

both work on my FreeBSD 5.4 32 bit athlon.

On my opteron and athlon64 machines with FreeBSD 5.4 or 6.0 (both 64 bit versions) the linux_enable="YES" in rc.conf has no effect, whilst 
# kldload linux

gives 
kldload: can't load linuxkldload: can't load linux: File exists

kldstat confirms that no linux.ko is present

I would be grateful for any advice as I have a 64 bit linux binary that i need to run

Thanking you,

Philip Lavers