If the Existing Member Login option is missing from the registration form, the link may have been removed from the New Member Registration form in the Signup Text section of WishList Member.
Also, be sure that the “Disable Existing Users Link” option on the membership Level is Not checked. If it is checked, it Removes the option for Existing Members to Login.
There is also an option in the latest versions of WishList Member (2.80 and higher) that allows you to use the “Improved” Registration Form Layout.
The “Improved: Registration form includes the Existing Member Login option by default.
You have the option of using the “Legacy” or “Improved” Registration Form Layout.
You can select the “Improved” or “Legacy” form in the Settings > Configuration > Miscellaneous section.