CRM_Begginer, have you switched the forms? For example, if I create a Marketing List using the first form, I see:

If I switch forms and select the second form:

Then I don't have a members list or + button:

Also, if you've created a Dynamic Marketing List and haven't saved it, you won't see the + either like below:

If these don't address your issue, then please provide some further information as to the steps you're taking to create the Marketing List and some screenshots would be helpful.
Thanks,
Megan
If this answers your question, please mark as Verified!