Dreamweaver can sometimes fail to recognize connections. I'll need more information to determine exactly why this happens.
Does the connection show from the Window->Databases panel?
Is the page you have opened saved into the same site as the connection file and named with a .php extension?
Have you tried closing and opening Dreamweaver to force it to refresh?
Can you successfully test the connection?
Have you installed the "Database and Server Behaviors" extension from the Adobe store?