Sync Strava rides
Currently assigning a bike to Strava rides is only supported from Strava itself. You first have to define a bike in Strava and then use it in your rides. Strava supports making a bike the default bike, so after you define it, your new rides will be automatically assigned to that bike.
Go to the Strava gear section, add a new bike and make it the default one. From now, your new rides will be assigned to that bike.
Optionally, you can go to the Strava activities list and edit your past rides to assign them to the bike you just created. If not, old rides will still appear as "No bike".
In the main screen of ProBikeGarage, press the menu button (three dots at the top right of the screen) and press "Sync with Strava". That will sync all your data from Strava and now your rides will be assigned to your bike.