You'll need to determine the spindle length of the current bottom bracket. There are many sizes; the correct length is determined by the crankset.

What's the model number of the crankset? You should be able to find the appropriate BB length with the model number on Google. There may be two lengths given. The shorter is preferred, but the longer one may be necessary if your bike is has a fat/aluminum seat tube, which is likely.

The current BB's length should be printed ont he BB shell, but you'll have to pull it out to see. Or, try to measure it with a ruler. Or, check your bike's specs. Or, just try putting the new crank on and see if it works; there's a small chance it will.

Resources:
http://sheldonbrown.com/bbsize.html
http://sheldonbrown.com/chainline.html

Edit again... You'll want to look up the chainline of the Nuvinci hub and adjust the BB length so the chainring has the same chainline. Generally, the chainring chainline will move one-half the change in BB length (since the BB length will be changed generally equally on both sides, yet only the right will affect chainline).