how to convert seconds in min:sec format

A versatile version is to use TimeSpan like this:

var span = new TimeSpan(0, 0, seconds); //Or TimeSpan.FromSeconds(seconds); (see Jakob C´s answer)
var yourStr = string.Format("{0}:{1:00}", 
                            (int)span.TotalMinutes, 
                            span.Seconds);

int totalSeconds = 222;
int seconds = totalSeconds % 60;
int minutes = totalSeconds / 60;
string time = minutes + ":" + seconds;

Just for completeness I will add an answer using TimeSpan (works as of .NET 4.0):

int seconds = 1045;
var timespan = TimeSpan.FromSeconds(seconds);            
Console.WriteLine(timespan.ToString(@"mm\:ss"));