How do I use only a custom menu on my Optimizepress membership sidebar

To do that you would need to create a custom menu in the appearance > menus section.

You would then need to go to appearance > widgets and the custom menu widget to the membership sidebar.  You would set this custom menu to use the menu you just created

Finally on the membership page you want to disable our menu and just use the custom menu, go to the membership settings and tick to activate the sidebar widget.  You would also need to add the following code to the custom css for the page (which can be found in the page seo and optimization options panel):

#sidebarlinks1{display:none;}

That should show just your new menu on the page you did that for.  Remember for each membership page where you just want to use your custom menu you would have to enable the sidebar widget and add that custom css to the custom css box to hide the normal optimizepress menu