What OS are you running? I'm running Reaper on a Macbook Pro with a TASCAM 1800 as well (cheers!), and I had the same issue with the 1800 - it was not recognized as an option for audio input or output. Turns out that was a known issue with Mountain Lion and TASCAM has updated the drivers for the 1800. If you're running Mountain Lion, you can download the newest 1800 drivers here on the 1800 product page on the TASCAM site (I can't post the link yet because I'm a noob).