Only Send A Message and Send A Group Message Message Types created and saved in Email Designs appear in Guest Messaging. Scheduled Messages only display Send A Message Message Types.
One-Time emails designs created in Guest Messaging are automatically the correct Message Type. However, these designs do not save and are not available to use in Scheduled Messages.
To change an email design's Message Type, go to Email Center > Email Designs > find the email design and Edit > use the Message Type dropdown menu to select the correct Message Type.