Mobistar has always blocked tethering on their network. Years ago their was a workaround but that was only for very old iOS versions.
Today, it is once again possible to enable tethering. This time without "hacks". Rumor has it this will be officially announced on monday but the functionality is already working. All you need is an up-to-date iOS version and the latest carrier update.
To obtain the latest carrier update, go to
Settings > General > Usage. Here a popup will appear asking if you want to install the carrier update.
Select update, and as soon as you have done that, the
Settings > General > Network > Personal Hotspot item should appear.
Configuration is quite simple. At the top of the page, enable the personal hotspot.
You will be asked if you want to use USB or Bluetooth/Wireless. USB requires iTunes to be installed on the computer, if you use this option, don't forget to disable the hotspot after use, otherwise the computer will use it every time the iphone is connected.
Wireless/Bluetooth are especially useful if you want to use a second device to access the internet (such as an iPad), or if you don't want to install any additional software on your computer.
The main disadvantage for now is the poor 3G coverage of the mobistar network. In most of the places all you get is Edge, which is enough to synchronize email but to slow for anything else.