you declare the variable min as a float....

var min:float = Mathf.Floor(ellapsedTime / 60);

And then you try to assign a string to it..

min = "0" + min.ToString();

So that's why it's complaining. You do the same thing with sec, though the typing there is implied rather than explicit.

unityscript doesn't let you change a variable's type on the fly like this. You'll need to use a separate variable of type string to hold these results.

Well, the problem is pretty much as the title says! You shouldn't be trying to assign the result of a 'ToString()' function back to the variable which you are calling it on. Instead, you should create new variables of the correct type, which you use for the GUI.

These bits are correct-

var sec : float = Mathf.RoundToInt(ellapsedTime % 60);

var min : float = Mathf.Floor(ellapsedTime / 60);

These bits are bad (trying to put a string into a float-shaped hole)-

min = "0" + min.ToString();
sec = "0" + Mathf.RoundToInt(sec).ToString();

Instead, make new string variables to hold this information

var minString : string = "0" + min.ToString();
var secString : string = "0" + Mathf.RoundToInt(sec).ToString();

On a side note, why don't you use an integer for the 'minutes' value? I don't think you'll ever want to have decimal values there, so there's no reason not to.