How can you show/download only 1 month of a budget? (Q Mac)

To show the 1-month budget view, simply click the "1 Month" tab near the top of the budget window. It's a rather goofy view, in my opinion, and it's not at all configurable for display or printing purposes. (The 1-month budget screen is a carryover from the 2010-era Quicken Essentials for Mac, which served as the code base for the modern Quicken Mac. The developers have not yet tackled re-coding this screen to be more in line with the current look of Quicken Mac.)

As for exporting, you can't Export the budget from the 1-month view. You need to switch to the 12-month budget screen, and then export. This will export all 12 months, but since you're opening it in a spreadsheet, it's fairly quick and easy to delete the other months if you just want to look at one.