For a good example of this module being used, see the Redmine Issues module by the same maintainer.
IMPORTANT: This module creates a user profile field for caching the logged in user's API key. This is only used if your Redmine version is prior to 2.2.0, before user masquerading via HTTP headers was available as an option, so if you are using 2.2.0+ then you can simply delete this field. However, if you are using versions prior to 2.2.0, be aware of two things:
You *must* apply this (very simple) patch to Redmine so it can look up user API keys (which is infinitely preferable to httpauth).