When you set up the game, make sure that the last box in the options tab ("No robots in the game") is unticked.
On the Timer tab, choose the minimum number of human players.
(If you want to play just with bots, set it to "1", and set the rating range to "0 points" so that no humans can join.)
Click the "New Game" button.
When you have the minimum number of humans, the "Robot" icon at the top left of the map window will be ungreyed. Click on it to select the robots you want.
Robots with a small number of games (or zero) will probably not be accepted. Try ranking the list by rating (click on the column title) then scroll down until you reach a bot with more than 1 or two games to its credit. If you hit on an inactive bot, you may need to exit the map and start again.
I usually only add one bot to a human game (if recruitment is slow). I know that PussyWhipt works - there are many others.