Do you have a BlackBerry data plan (BIS)? If you have a generic data plan, you cannot use the native RIM browser or any other native apps (Facebook, MSN, or AppWorld).
Posted from my CrackBerry at wapforums.crackberry.com
What country are you in?
What carrier/service provider are you using?
Does the carrier/service provider offer BLACKBERRY Internet Service Data Plans (BIS)?
Do you have such a plan active on your account?
I found your carrier's Web site. This is the English version. It appears that they do offer BlackBerry Internet Service, so you will need to contact them and have them add a BIS plan. Then your BlackBerry browser, Facebook app, and MSN app will work.
Without an active BlackBerry Data Plan from your carrier, you will nto be able to use BlackBerry Data features such as the BlackBerry browser, BlackBerry Messenger, push email, Twitter app, Facebook app, Google apps, BlackBerry App World, etc.
It appears that they do offer BlackBerry data, however. Mobitel.si
You will need to call Customer Service for your carrier/service provider and ask them to add the data plan to your account. You might also be able to do it at a Mobitel store.
It will likely cost you an additional amount above what you are already paying.