You need a membership (not just credit) and an internet add-on (if the membership doesn't include data) for internet access to work.
You also need to configure your internet/APN settings..
Try the online configurator:
http://community.48months.ie/t5/Need-Help/Internet-not-working/m-p/8361#M2747
If that doesn't work and you're on Android then download the Tweakker APN app from the Google Play Store over wifi and run it:
https://play.google.com/store/apps/details?id=com.tweakker
If that doesn't work then you'll have to try configuring them manually:
Internet Settings
Remember to enable data roaming.
Hope this helps.
