Word permits a shape (text box) to extend beyond the edge of the page. You can not view an area beyond the edge of the page. You cannot scroll to it.
You have three options to type there and see what you are typing as you type it:

• Temporarily change the layout to landscape.
• Temporarily change the column widths so you can see all the columns.
• Temporarily change the page size to 11" x 11."

Another option would be to copy the contents of a cell and paste it outside the table. Edit it there, cut it and paste back in the appropriate cell. This is what I often do when mail merging labels (which are tables).

Many thanks, Charles!