When creating a new campaign, in the text editing tab - click on 'Add Another Variation'

Once you have another version, you can decide how to conduct your split test.
If you'll select a certain percentage within your list, the system will divide the versions between this percentage.

You can also select the time frame for this test and after that test, the system will send the rest of the data to the winning version which can be based on clicks or conversions.