XP plays a huge role in MergeCrafter and you will need to increase the amount of XP you can hold to unlock more land as you progress. The default XP cap is 350.
There are 2 different stages of each XP Jar (Small and Medium). You simply merge 3 XP jars of the same kind into each other to get the Medium variant.
As soon as you craft your first Oak Axe, you have access to the Crafting Table.
Navigate over to the Storage category where you can craft 4 kinds of XP Jars. You unlock them as you progress.