From the Apollo's manual:
When connecting via XLR you will always need to set the corresponding Apollo input to "MIC". When a particular input on an Apollo interface is set to MIC the Apollo takes signal from the XLR connection corresponding to that channel. If you have your input set to MIC but are connecting via 1/4" TRS, or if you have your input set to LINE but are connecting via XLR, that would cause no input to show on the Apollo.
Just buy a XLR to TRS cable.
Ha! I never thought about that and it makes total sense! I just bought XLR>TRS cables to output the Kemper into the Apollo!