Why are cursor keys in Excel moving the viewport rather than the active cell? Excel for Mac (2011) should move the currently active cell when I use the cursor keys. Currently, it is moving the viewport in the document. Is there a way to switch this back to the default setting? I cannot find anything in Excel's preferences.
This sounds like Scroll Lock is somehow activated on your copy of Excel. IF you have a large keyboard with an F14 key, ShiftF14 can turn on and off Scroll Lock. If you don't have such a keyboard, or access to such a keyboard, it gets more complicated.

Microsoft support posted this article about the various methods of deactivating Scroll Lock. They either involve connecting a keyboard with a F14 key or removing your Excel preferences file.