If you want to play in DIDO you have to start a table and your partner sits opposite you or vice versa.
If you have no pre-arranged partner, then you can advertise for a partner;
OR sit opposite an advertising player at a DIDO table
OR ask a player to play.
Press Invite To Play in the profile of the player you want to Invite
Make sure that you are NOT seated when you do so.
If your invite is accepted then YOU have to start a table and the invited player will be auto seated opposite you.
You cannot manually sit as opponent. The server will get you opponents.
To start a table press Start a DIDO Table top left.
When both are seated, agree on SSD and the server does the rest.
See article on the home page or click this direct link