In Dynamics CRM 2011 you can create a Many to Many relationship between entities. This is great for associating entities to multiple reference data items – for example Projects to Project Locations, or Orders to Products.
When you go to filter an Advanced Find by this related entity you may find that it is missing. You need to ensure when you create the relationship that you set the Display Option both sides to Visible – as by default they will be created as Do Not Display.
For a more detailed explanation with images, check out this post from Allan Varcoe. As Allan points out, this automatically creates a new item in the Forms navigation pane which might not be what you want. This can be manually removed by customising the form navigation and publishing.