This is more of a helpful tip than anything… since this is a problem I ran into today.
If you have potting mix that has dried out and is now hydrophobic…put it in a big container (I like 5 gallon buckets) and add boiling (or near boiling) water and stir with a stick, etc. The hot water seems to penetrate better and you end up with a wet potting mix again. Note to self: Close bags of potting mix better!
Wait until the mix cools down before you use it!