Opportunity Product Quick Create Form not working

I want to use a quick create form to add opportunity products rather than the OOTB inline editor. So I removed the OOTB grid, added a new sub-grid looking at the "All Opportunity Products" View and created my Quick Create Form.

Pressing the add button on my new grid and absolutely NOTHING happens. I was expecting a lookup box to appear but it doesn't.

Any advice would be appreciated.

Cheers,

John