To specify the table to join the current table with, click on the join button. This will display both the history view and the table view on the left. Either click on a table in the history view or a table on the tables list on the left. This will then join the current table with the selected table.
Specifying join conditions
Simply select the columns on the left and the columns on the right that should match for a successful join. If you have multiple conditions, you can also specify whether a join should occur if all conditions match, or if only some conditions match.
When you join with a table, you will see all columns from the table you are joining with. If you only want to see a few columns, use the "Select Columns" operation after you join.