I´ve looked at the monotouch samples and tried:
var range = new NSRange(selectionStart, 1);
NumberTextField.SelectedTextRange = IndexedRange.GetRange(range);
But this causes this exception at runtime: Objective-C exception thrown. Name: NSInvalidArgumentException Reason: -[iOS_IndexedRange asRange]: unrecognized selector sent to instance 0x7d9bd690
I also looked at a lot of objective-c versions but i cant figure out how to do them on monotouch.