Shortcut to move one screen to the right/left in Excel
Although it seems counter intuitive not to use the left/right arrows, (that is a subject for giving Microsoft some feedback) using the following key combinations work for Excel 15.xx for Mac:

Using Built-in MacBook Keyboard:
Page Right: fn+⌥ option+↓ down arrow
Page Left: fn+⌥ option+↑ up arrow
Using External Apple Wired Keyboard:
Page Right: ⌥ option+page down
Page Left: ⌥ option+page up

As noted in the OP's screenshot.