Webform CiviCRM: Issue with Membership Options Not Displaying
Look for them on the Start page - and make sure you have configured the Membership (under the Webform tab) - as a user select. Note here you can also decide if you want live options or static - and if you go static you can decide which ones are available.

Add on:
Before all this you will want to get into the Membership section under the Webform -> CiviCRM tab.