Quite close to MBK. You have two ways of getting there from the IC:
1: Walk on the overhead walkways to Paragon, from there cut over the Siam Square- which then connects with a glass bridge to the Discovery centre on level 4- exit out of Discovery Centre on level 3 in the direction of the signs for the BTS (Skytrain) and then follow the overhead walkway into MBK. This is often the fastest and I find the easiest if you don't mind a walk of about 15 minutes (of which 7 are air conditioned and 8 are outside but on the overhead walkways).

2: Take the BTS to Siam, Change at Siam to National Station- at National Station walk out and follow the signs to MBK (clearly marked). While this option sounds easier it is not because changing at Siam will be very busy indeed usually.

I would take option 1 unless the rain was torrential as you are covered almost the entire way anyway.