So i just got myself Armor+MX (Mid-tower), Now i wonder how to power the large side fan ?
It has small white two pin connector, and wonder where does it go? into the three pin sockets found on mobo ?

Incase, you have an armor, plz inform how have u connected your fan ?

PS: In sum old thread (I googled it), sumbody was saying the side fan gets connected to one other connector already present on the case, is it true ? Am i missing something...

The fan connects to the motherboard, not to the case. The two pin connector is an older style, back when it was just the positive and negative wire. The 3 pin style on your motherboard adds another pin to add fan speed monitoring/changing capabilities. You should be able to get a two pin connector onto a 3 pin connection point, just make sure you look at the motherboard manual and make sure you connect the positive and negative correctly.

Other options include
2.) gett a fan control that has a two port plug,
3.) cutting the plug and replacing it with a 3
4.) getting a 2 pin to 3 pin adaptor

If i connect via mobo - How to make sure i connected positive & negative correctly ? I dont knw how to check tht - but can manage if someone tellme or provide a online guide.

Also, is it possible to cut the original 2Pin plug and attach a new molex or 3pin plug to it.... Then i guess i can use fan without probs... -- if i do this what precautions do i need to take so tht everything goes smoothly ? Also inform do i need to solder the wire into new plug ? Or it just get attached ?

To check the positive and negative, look at the cord coming from the fan, there should be two wires in the plug, one will be black and one red (or another light and dark color) the black is negative, red is positive. Then look at the manual for the motherboard and for each fan socket it will tell you the order of the 3 pins on the motherboard.

Yes you can cut the connector and put another type of plug on. If I'm not mistaken I think my armor case came with a 2 pin to molex connector. No soldering should be required, normally you would just cut the wire, attach two metal pins (they would come with the new socket) and they slide into the socket and lock in place...