I'm not a Mac user but a quick look on the Alesis site suggests you may need to installed specialist Mac drivers, available here: MultiMix 8 USB 2.0 Integrated USB 2.0 audio interface and mixer
Edited to add...it wouldn't take a direct link but if you click on "Documents and Downloads" you should get what you need.
Thank you very much for the reply Bob!
Unfortunately, the most recent driver that Alesis lists to download doesn't do the trick. I'm running Mavericks on my iMac (for anyone that works on Apple computers), and the driver updates on Alesis are for Mountain Lion.