Don't know if this is what you mean? Otherwise, you will have to explain a little more...
time=String.Format("{0}:{1}",min,sec);
or if you need the exact format:
time=String.Format("{0}:{1}",min.ToString().PadLeft(2,'0'),sec.ToString().PadLeft(2,'0'));
[Edit: Matt Heffron]
Use format strings:
time = String.Format("{0:D2}:{1:D2}", min, sec);