The control has a property MembershipProvider - Gets or sets the name of the membership data provider used by the control that you can set to the provider you want to use.
When you set AllowSelectMembershipProvider you should see all providers which are defined for the application through web.config (for ASP.NET SQL provider) and SecurityConfig.config for Sitefinity's specific providers. To see your provider name in the drop down you should have at least one user created in this provider. The dropdown order is constructed by the result of UserManager.GetManager().Providers; Changing this order will require a custom control.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items