Using the Mac drivers, you pick the paper source when you pick your paper size. In most applications this is part and parcel to the print dialog; in Lightroom, you instead configure this from the Page Setup dialog.
It's a little clunky because this isn't the normal way to pick a paper size, but still not too difficult once you know what you're doing.
1. If your R2880 isn't already selected, select it under "Format For". The dialog will otherwise only display universal sizes, not the printer-specific ones you need.
2. Under "Paper Size", select both the paper size and feed path you'd like to use. "Sheet Feeder" is the primary automatic sheet feeder. That's the one you can load a stack of RC paper into. Other options vary with the printer model; "Manual - Roll" is typically used for both roll media and the single-sheet feeder (which feeds through the roll path).
Not all feed options are compatible with all media types, but you should get a more specific error message (along the lines of "selected paper can't be used with selected path") in that case.