Something like this:
Code:
FamilyInstance familyInstance = dbDoc.Create.NewFamilyInstance(InsPoint, symbol, StructuralType.UnknownFraming);
string coordinateString = InsPoint.X.ToString("F2") + ", " + InsPoint.Y.ToString("F2") + ", " + InsPoint.Z.ToString("F2");
familyInstance.get_Parameter("XYZ").Set(coordinateString);
I'm assuming InsPoint is an XYZ point, if it's not then adjust the code accordingly.
double.ToString("F2") will cause the number to appear with 2 decimal places
The above code will also need to take place during a Transaction.