Well, first of all the DLink DIR-450/451 isnt mentioned in the Table of Hardware, and the DLink site itself does not mention linux, so you should check whether the router is actually capable of running OpenWRT. Did you get an GPL copy with the router? Is there any mentioning of linux in the manual?
You should try nmap to check if the router could be running linux. Next step would be looking at the hardware and identifying as much of the internal system as possible (e.g. wireless chipset, cpu, amount of ram/flash, etc).
If it is an unsupported hardware platform or not enough ram, you are most probably out of luck.
Edit: Just checked the svn log, and it seems that OpenWRT does support the DLink DIR-45x, the Table of Hardware is just outdated.
Most probably you need to check out kamikaze from trunk and build it yourself.
(Last edited by KanjiMonster on 15 Dec 2007, 15:12)