External monitor won't work with 12" MacBook

I recently bought a 25" 16:9 2560 x 1440 WQHD Asus Monitor that was recommended as an external monitor for my new MacBook with Yosemite 10.10.5.

I have an Apple-branded USB-C Digital AV Multiport Adapter that includes an HDMI port. I connected this adapter to the Asus monitor using the HDMI cable supplied by Asus. As far as I can tell, the cable is firmly inserted in both the Apple adapter and the HDMI port on the monitor.

When I turn on the monitor, atolls me there is no HDMI signal. My MacBook appears to try to connect, show the monitor's manufacturer and model number then my MacBook screen begins jumping between the monitor's screen size and the regular 12" size. Nothing I have been able to do will stop this erratic behavior which is impossible to watch for long.

It seemed like this would be a simple project, but it's not working out that way.