Hi sattarkf, I've actually given a detailed explanation on how you can do this in a previous post here
. As for connecting using the default network manager, it is actually quite simple. you just right click on it choose edit connections> mobile broadband and add. another window will open with you device in the drop down and you basically go forward from there. if you have already configured your device earler, you will know what your apn and plan is.
The only problem I've found with network manager is that it does not connect always. I have to wait for 5 minutes after boot up and plug in before my device gets recognised, and even then there is a 50:50 chance that it will not connect. Join air though connects first time, every time and takes only a few seconds to scan for and locate the network.
Hope I've helped.