Examine your microphone and determine whether it needs phantom power or not. If so, your external microphone preamp will need to provide it, since you are bypassing the phantom power in the Mbox 2 Mini.
Connect an XLR cable from the microphone to the female XLR input of microphone preamp. Engage the phantom power on your microphone preamp, if needed.
Locate the outputs on the microphone preamp. If they are 1/4-inch TRS outputs, run a 1/4-inch cable from the preamp directly to the TRS Input 1 on the Mbox 2 Mini. If they are XLR outputs, connect the female end of an XLR cable to the output on the microphone preamp (see link in Resources). Run the male end of the XLR cable into a female XLR to male TRS adapter. Plug the adapter into the TRS Input 1 of the Mbox 2 Mini.
Press the "Mic/DI switch" on the back of the Mbox 2 Mini to change "Input 1" to the TRS "DI Input," instead of the XLR input.
Adjust the gain on the front of the Mbox Mini 2 for Input 1 to the desired level for your recording.