Before you're able to hire a Diggerling you must first reach level 26, once you've reached level 26 a short video which will introduce you to the Diggerling character will play.
To hire him you need to visit Arthur's Inn, once you've hired him the achievement will unlock straight away.
Lastly, you're awarded experience in this game each time you achieve a feat, for example, the first time a Whirlm visits your garden, or growing a daisy to full maturity, gaining experience in this game is easy, there's plenty of awards out there for you to complete and earn yourself some experience. The small daisy in the top right corner of your screen represents your current level, the number in the middle being your current level and the petals around it being your progress toward the next level.