I think from your description that you want to insert a new field into an existing table rather than inserting a new row? A row would contain actual data, whereas a field describes the data that you are holding. Example:
You have a table named products and you want to store product_id, product_price, and product_description. product_id, product_price and product_description are all fields.
A row in that table would be (e.g.):
00015 | 15.99 | Pack of 6 widgets
If I'm right and you want to add a new field to an existing table, you need to do the following:
Click on the relevant database
Click on the relevant table name
Scroll down to the option labelled "Add new field" and enter the number of fields you wish to enter and where you want to add it/them
A screen will appear that lets you choose the field name and specify the data type and any other options
Hope this helps!