<I inherited a non-working fork lock so I ordered a replacement and can't seem to figure out how to remove old one. I'm sure I'm missing something simple,>

Have you removed the grub screw?

The steering lock is normally retained by a grub screw (21-0578), and is then sealed over with a 'sealing washer' (97-2266) to prevent tampering.
Look for a small circular indentation at the front of the yoke immediately ahead of the lock, also look at the fork diagram (pdf p.48) in the link below.

http://www.classicbike.biz/Triumph/Parts/1970s/1978-Triumph-Parts-Manual-Bonneville-Tiger.pdf
Grub screw[9] and sealing washer[10] see pdf pages 48 & 49 (1978 parts book, but the lock assembly is the same for a '76 model)

If you drill out the sealing washer and remove the grub screw you should then be able to remove the lock by knocking it upwards out of the yoke?