What EXACT version of the OS, of Photoshop (CS5 is too vague, we need to know if you have applied the 12.0.4 or 12.1 updates), what version of the printer driver?
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I had this happen once also on my Epson 2400...
but can't remember the exact fix.
Do you have paper loaded in the rear feed? Or do you have rear feed chosen for your paper type?
Check those. And I would also do a shot gun fix of rebooting printer, computer and trashing Photoshp prefs.
The paper and ink options are managed entirely by the printer driver, not the application.
Today I had your exact same problem. Epson Customer Service gave me the solution.
Go to the Mac Apple menu to System Preferences, to Print & Scan, to your printer (EPSON Printer Stylus R2400), delete it, and then add the same again, and Ready, now you can select your paper. Have to do this every time you change from Photo Black to Matte Black and viceversa.
The problem never was Photoshop, is EPSON. This comment are in the wrong place, they should be in a Printers or EPSON forum.