In the Report Designer, the buttons above the columns and rows dimensions lists let you toggle between the standard Member Selector and Tuples.
The Axis Set Editor enables you to choose a combination of members (tuples) on columns or rows, as well as an Excel member set.
Display the Axis set editor
The Axis selector doesn't display any tuple when the grid hasn't been refreshed yet.

Once the grid is refreshed, the Axis Selector lists all the combinations of members available in the grid for the axis.
Remove tuples
Select the tuples to remove, then use the delete button.
Switch back to the standard selector and select OK to change the axis selection to the cross join of members of all the dimensions.
Add tuples
Select the ADD button at the top to add a new combination to the list.
The new row can be moved anywhere by dragging it to the desired location. The dropdown combo boxes let you select the desired member for each dimension:
Selecting Duplicate from the right-click menu can be a quicker way to add a new combination, particularly when many dimensions are used on the axis.
Convert tuples to an Excel member set
You can output the list of tuples to an Excel member set driving the axis. Undo this by using the Back button on the grid ribbon.