We pre-install Adminer on all LAMP based appliances and you'll find it on port 12322 via https. So if you're already logged into Webmin, just remove everything from the end of the URL and adjust the port (Webmin is port 12321, so just change that to 12322).

Unfortunately, I don't recall the exact steps to upload a DB via Adminer, but I imagine that it should be relatively self evident. FWIW, personally, I'd just use the commandline (via SSH):

mysql -u root DB_NAME 

(replace 'DB_NAME' with the database name and '/path/to/file.sql' with the actual path to the SQL file you wish to import)

If you prefer phpMyAdmin, installing that is certainly an option, but it will only work via a sub-directory (or perhaps sub-domain too?). It won't stay bound to a port unfortunately, which is the main reason why we switched to using Adminer (besides the fact that it's smaller and faster).