The Sales Performance dashboard has five columns.
The built-in UI tools can only edit dashboards up to four columns, even though the system can render more complex ones.
To modify the Sales Performance dashboard you would need to to export it in a Solution, modify the XML and re-import.
I suggested on Connect a while back that this limit be increased, especially now we have a widescreen type experience without the left navigation. You can vote for the suggestion here:
https://connect.microsoft.com/dynamicssuggestions/feedback/details/808859
Hope this helps.
Adam Vero, Microsoft Certified Trainer | Microsoft Community Contributor 2011
UK CRM Guru Blog