First, make sure you have version 1.31 of the US2400 driver and firmware for Mac OSX. You can get the download, along with installation instructions, here:
www.tascam.com
Start up the US2400 in Native mode, as this is what you need for Logic. To do this, hold the [SEL] + [CHAN] buttons while powering on the 2400.

If it still isn't working, quit Logic and go to MacHD>Users>(Home Directory)>Library>Preferences>Logic and delete the file com.apple.logic.pro.cs, the re-launch Logic.