To query by taxonomy you need to know its query_var that is used in its register_taxonomy() call. It defaults to taxonomy name so usually equal to that.
Then you use that info in query argument:
query_posts( array( 'query_var' => 'term' ) );
See Querying by taxonomy in Codex.
It is hard to suggest more without specifics of your taxonomy and where you need it done (template, standalone function, etc).