First of all, this client is not supported by Spotify, as is clearly stated on the web page you're linking to.

Secondly, since it's most likely compiled for X86, and not ARM, getting it to work on the N900 will not be easy. There is some work going on to use X86 binaries on the N900 but I have no idea of the status.

If Spotify supplies an ARM binary, things are different.