You may find that all of the rows in the various resource tabs of the EPE become filled with auto-populated values from the IP you add on the IP tab.
The EPE is designed to automatically add a new row if you have fully populated all of the rows available in any resource tab. However, if you have no resources entered, and if you use the IP tab first, it is possible the resources added by the IP tab selections will completely fill all of the available rows on each of the resource tabs. This will prevent you from being able to enter any further resources used by your design.
Prior to entering any selections on the IP tab, use at least one row in each of the resource tabs. For example, you can enter a value of "1" in the first row of the Logic tab for # Combinational ALUTs. Continue to use enter at least one device resource in the remaining tabs that will be affected by the IP you select. By doing so, the EPE will continue to auto increment the row count on each tab allowing you to enter your own design resources along with the auto-entered resources from the IP tab.