I'm assuming that you have the rather annoying modernised and reduced version of the Genie simcard! Well, the 'new' O2 Online sim!
To be able to access the net, you need to visit O2.co.uk and search for a section regarding receiving the service settings on your phone.
Once you locate this, fill out the form and then a configuration file will be sent to your mobile.
Save this file, and then key '0' is usually a shortcut for accessing the net. Your 300 minutes will start from when you access it.
Are you using 915000 or the 07 number?