Go to the right hand side and click on the database name. This will bring up the details for the database in the right hand window. Click the SQL button at the top of the window. This opens a box, titled "Run SQL query/queries on database. You will then need to type the SQL command to delete the field from the table into the box. It would look something like this: ALTER TABLE 'this is where the name of the table goes' DROP 'this is where the name of the field goes' . Then when you are done, you click on Go. A pop-up will ask if really want to perform the action. Click OK. The command screen will then confirm that your SQL query has been successful. You have just deleted a field from a table.
For more information about SQL syntax, visit the MySQL documentation available here. Just click on the icon under the phpMyAdmin name, that has the red SQL on it.