I am having same trouble, but also read in the documentation here:
docs.abantecart.com/pages/design/menu.html
that Link to Page at bottom of menu entry is supposed to work. It does not stick after saving.
Also my the Content pages should have an ID field on them (read-only of course) so that we can see what ID number they are instead of interpreting the URI
eg /index.php?rt=design/content/update&content_id=0_1&s=admin&token=blahblah
Unfortunately, the ID of 0_1 doesn't work for me either, nor 0 nor 1....