You can find your login details in JVZOO - where you were returned after purchase. You can also access your details via customer.jvzoo.com - the JVZOO customer portal.
Alternatively - look for your welcome email form email@example.com
If your details do not seem to be working please ensure that they are your login details for the administration area of members.io
You can have them resent to you by clicking forgotten details - https://membersio/auth/login/