How do you add a "Comment/memo" column in Quicken Deluxe for Mac v 6.5.1

Unfortunately and somewhat inexplicably, it's not possible to set the Memo column to appear in the Bills & Income screen.

There is an existing Idea thread (request for feature/enhancement) for adding the Memo field to that screen; you should click on this link to visit that thread and add your vote for it (in the yellow box under the first post).

Meanwhile, you can set your account registers to show future scheduled transactions, and to show the Memo field, so you might want to do that. To add columns, click the Columns icon in the bottom menu bar of any register. To set a register to show future scheduled transactions, click the little "v" icon to the right of the little clock icon near the top of any register (under the Search box), and select one of the options to control how far into the future your scheduled transactions display. 

Note that these register changes apply register by register, so you might need to make the same changes to multiple registers. Also note that these same features apply to register of account groups — Cash, Credit, Banking, All Transactions, etc. — in the left sidebar. So if you click on Banking and make these changes, what you see in the future should pretty much be the same as what you see in the Bills & Income screen, with the addition of the Memo column.