In the main navigation you can create a drop down menu.
Go to 'Layout' > Click on the 'Navigation' tab.
Follow this three easy steps:
1. In the main navigation, add the link that should be shown as a drop-down menu.
Click on 'Add link' > Enter the name, e.g.: Category.
If you do not want this link to be clickable, select 'URL' in the URL field and type "#".
2. Create a second navigation with the exact same name as the link you just created.
3. Add all relevant links to your navigation. They will be displayed as a drop down menu.
4. Check out how your drop down menu looks like in your store: