Depends on the version of the add-in you're using.
The Model View you're referring to was introduced with the SQL Server 2012 version of PowerPivot for Excel (v2). There's a button in the top right on the Home tab labeled "Diagram View". See
this page on Technet for a visual and details.
If you're using the first version of PowerPivot (the SQL Server 2008 R2 version that shipped in 2010), that one only had the Create Relationships and Manage Relationships buttons that take you to the dialogue you describe above.
If you're still on v1 and you're in an environment where you can upgrade to v2, I'd highly recommend it. Lot of improvements and enhancements.