How to associate a category to a post wordpress?

c# wordpress xml-rpc metaweblog

Another thing you could use is the wp.newPost method:
http://codex.wordpress.org/XML-RPC_WordPress_API/Posts#wp.newPost

This uses 'taxonomies' instead of categories.

I'm currently updating the JoeBlogs Wordpress wrapper to support taxonomies (categories)
https://github.com/alexjamesbrown/joeblogs