I have downloaded WriteNCite from RefWorks but I can't see the referencing style required by my school in the Referencing Style list.
If you have a short list of Referencing Style options, then it's possible that your RefWorks account does not link to the University-affiliated resource.
To do this, make sure you're logged out of RefWorks in your Word document(s), then click Log In in the RefWorks ribbon.
The log in window should show an option with a link to Use Authorization Code. Click the link.
In the new window that opens up, paste in the Authorisation code from RefWorks. You will find it under Tools > WriteNCite.