OneNote does tables a little differently than Microsoft Word. Here are some clever tricks to master OneNote tables.
Type some text, press tab: Voila, a table!
If you only have one row of a table:
To add more columns: Keep tabbing
To add a row: Press enter at the end of the last column
If you have many table rows:
To add a row to the end: Press tab in the last cell
Whether you have one row or many, you can always press Ctrl-Enter to add a row below the current row