Just by selecting “USB CarPlay” or “WiFi CarPlay” from the main menu, either via touchscreen or using the iDrive controller, and it takes a couple of seconds to load usually.
Of course there are any number of reasons why our experiences with it are different - installation issues, Andream version, iDrive version, etc. (my car is an E60 so likely different software to yours anyway). As I said the car had this installed when I bought it so I know nothing about it really other than it works well for me, so it’s a pity yourself and others have had issues.