I'm going to assume you're talking about the Bandai America release. Basic gameplay involves shaking the device to get steps and walking along a map with your chosen partner Digimon. Every now and then, you'll get into a fight. You'll power up your Digimon by swiping a card; the type of attack you use depends on the speed at which you swipe the card. Any card works, as long as it's sturdy enough. You have no control over evolution in battle. Instead, you automatically evolve as you advance through certain areas of the map.
The US release also came with a PC game. This involved uploading the Digimon from the Digivice onto your computer and playing a spectacularly slow and un-fun fighting game with them. The game was originally meant to be played online, but online play is no longer available. The game also had a Digimon Encyclopedia that was really the best part of the whole deal.
IIRC, the two side buttons are used to scroll through options, the bottom to confirm, and the top to cancel. I might have the top and bottom buttons mixed up...it's been awhile.
Is there any specific issue you need help with?
And someone needs to move this to Digimon Gaming. ^^;;
Kind of understandable since they use seriel ports.. but still... ;-;