Hi,
Wait, you have separate WordPress installations within subfolders? That is the problem then, as that is not a Multisite installation – but those are separate WordPress sites.
To create a multisite network, follow the instructions on this official WordPress tutorial: Create a Network
I am not sure if it is possible to import existing WordPress sites as multisites though, I have never tried it yet. A multisite WordPress is basically one single installation, witch a shared database.