Go to Subscriber Management in the DashBoard menu or front page of Listserv. You should have selected you list, here it is TESTFLAG. Then select "Subscriber Management".
You can go to "bulk operations". Click appropriate option Add and find your text document in the Browse. Hit import and all addresses should be added.
Here is an example of a "text" document with information to add in bulk.
Or just the email addresses are fine.