Try clicking on the cell and then under 'Property View' (bottom right) where it says 'Text' click the white blank box.
When you do this you see a box containing 3 dots pop up in the left. Click that.
This should open a popup requesting text. Try typing in that.
A forum search for Mac shows that there are strange problems on Mac OS in rare cases. Usually, deleting the profile directory helps. It is located at /Users/[USERNAME]/Library/yWorks/yEd.