Hi rlovi,
After you select the race type (Practice for example), click on "Configure" at the next screen. Click "Next", under the "Candidates" box, there are two selectors, change the bottom one to "human". Your configured players should appear in the "Candidates" box. Remove any humans from the "Competitors" box (select it then click on the 'X' below), then add the player that you want from the "Candidates" box (select it then click on the '+' below). Click "Next" until "Start" appears on the far right.
Click "Start". You will only need to do this once for each race type (Practice, Quick Race,etc) as the game will remember the choice for each race type.
Here is a link to a video that Simon posted. There may be some minor differnces in the screens.
https://www.youtube.com/watch?v=i8Db_RMNx4A