At first, you should download jabber plugin for Miranda.
To make Miranda support openSSL, you can find dll files at here (Download)
Extract ssleay32.dll libeay32.dll to your Miranda Installation directory.
What you need to fill in on Options–Network–Jabber:
Username: your gmail account
Password :your gmail account password
Login Server: Gmail.com
Port: 5223 with "Use SSL" checked
Check the "Manually specify connection host" and fill in with:
Restart your Miranda and have fun.