To be able to play you need 2 things:
1. A battle.net account with an active Diablo III beta licence.
2. A system configuration that's compatible to the Diablo III beta client.
More configurations are likely to be added later on for the beta (with upcoming patches), but till the patches are up, those with a configuration not compatible won't be able to run the client.
Try downloading the latest graphics card drivers, that might do the trick.
1. A battle.net account with an active Diablo III beta licence.
2. A system configuration that's compatible to the Diablo III beta client.
More configurations are likely to be added later on for the beta (with upcoming patches), but till the patches are up, those with a configuration not compatible won't be able to run the client.