Firstly, create a List Button under App Setup > Customize > Opportunities > Opportunity Products > Buttons and Links. Uncheck "Display Checkboxes (for Multi-Record Selection)", as [last time I checked][1] you can't have list controllers for OpportunityLineItem.

![List Button][2]

Then, when editing the Opportunity page layout click the small spanner above the products related list. The Buttons section can be a bit hard to find and will need to be expanded up from the button of the dialog. From there you can add your new button.

![Related List Properties][3]


  [1]: http://www.fishofprey.com/2012/06/salesforce-custom-list-buttons-for.html
  [2]: https://i.sstatic.net/aYTTv.png
  [3]: https://i.sstatic.net/HdsR8.png