How do you use the Arduino IDE with a Chromebook.

Not yet you cant.

There are plans for the CREATE version on android but for now you are out of luck sorry about that.

Thanks. I saw some Arduino apps on the Google play Store. But I'm wondering if I could get an Android OTG USB ADAPTER, and program an Arduino board using the USB hooked up to an Android phone?

There are some that may be of use to you but they are not the official ones and as such you may need to go directly to the app's support site to get details of what and how they work.

I dont think I have seen anyone mention getting them working with OTG but stranger things have happened and you would have to do your own research for that aspect.