First of all, this is all going to depend on what you're operating on. If you're on a OTA update (stock ROM), it may or may not be included (which I can't tell you if it is or not, based on that I've never used a stock rom), but if it is included, it'd be under the developers options menu. Custom ROMs, like the one that I'm using, do include the option to switch to ART on the same device you're asking about (specifically, grouper, or the 2012 Nexus 7), but that's not to say all custom roms will include the feature. It all depends on what the developer of the ROM has included in it.
Google's OTA update might not have this feature included because of, at this time, it is still a work in progress. If it's not in the developer's options yet, be patient, wait a while for them to implement the use of ARM more universally, and then make the switch. If you don't know how to get to the developer's options menu, then maybe your knowledge base of android should be aged some before rushing into asking "why it's not included" when it very well could be.