Hi MM,
You can't add times (actually Date and Time values. You can add Durations, and you can add Durations to a Date and Time value.
If you enter the characters 12:45 into an unformatted cell, numbers will understand that as the Time part of a Date and Time value, and will likely show the result as 12:45 PM. Although it is not displayed, that D&T value also includes a Date part. Entered today, the full value would be August 17, 2013 12:45 PM.
To make Numbers interpret that 12:45 entry as a Duration, you need to either enter it in an unambiguous format (12h 45m), or format the cell as a Duration before entering the value.
When you have a column containing only Durations, you can add them using SUM(B) (where B is the column containing the values).
How the sum is displayed is controlled by formatting the cell containing the SUM() formula.
Open the Inspector, choose the Cell Inspector ( 42 ), then choose Duration, use the slider to chose the units, and use the pop-up menu to choose the format. 
Regards,
Barry