You can signup to beta test Enhanced SMS & Caller ID + by joining our Testers community.
To join our community, all you have to do is click here.
Then once you are accepted, you can sign up to beta test the app by clicking here. You should then see a page similar to the one below.
At any point, you can leave the test (which should revert you back to the market version of the app). At the very bottom, you can find the link you should click to download the test version.
At this time, beta testing is only open to those that have bought the app through the Google Play store.