Not sure how you're trying to do the install, but best advice I can give you, is to try and follow the step-by-step instructions on this blog:

http://blog.jakgibb.com/2013/03/28/a...stall-on-n900/

Not perfect, but the best you're going to find I think....

Other choice is to spend lots of time reading through the posts in this thread from about page 25 on.....but I'd recommend doing fresh install following the blog....

As Skry noted earlier, this is not the easiest install and I'd add even if you've installed Arch before on other computers....

Just my 2 ¢.....