In order to add manufacturers do the following:
Click on CATALOG > ATTRIBUTES > MANAGE ATTRIBUTES like so:
Then find the MANUFACTURER attribute and click on it.
When in the manufacturer attribute, click on the left hand side tab ‘MANAGE LABEL /OPTIONS’. You will see a big list of already existing manufacturers. They are listed in alphabetical order. In order to add another manufacturer, click the blue ‘ADD OPTION’ button and enter the manufacturer in the newly created space. Once you’ve added all the manufacturers you want, hit the ‘SAVE’ button.