I am trying to record my roland td-11 electronic drum kit. I have plugged an auxiliary cord into its mono output port and into the audio jack of my iphone. The only recording option, however, is through my phone's microphone.

Is this not a valid way to record? If not, how might I easily record my drums using the app?

Ah yes, the headset jack is a special type, so you'll need some kind of adapter. Highly recommended is a USB interface though, so you can record in stereo and without crosstalk. Some ideas here: http://www.harmonicdog.com/inputoptions/