To enter time durations in the [mm]:ss format and then sum them up in Microsoft Excel, you can follow these steps:

1. Select the cells where you want to enter the time durations.
2. Right-click on the selected cells and choose "Format Cells" from the context menu.
3. In the "Format Cells" dialog box, select the "Custom" category.
4. In the "Type" field, enter the following format code: [mm]:ss
5. Click "OK" to apply the custom format to the selected cells.

Now you can enter time durations in the [mm]:ss format in the selected cells. To sum up the durations, you can use the SUM function. Here's how:

1. Select the cell where you want to display the total duration.
2. Type the following formula:
   Formula:

   =SUM(A1:A10) 

   , where A1:A10 is the range of cells containing the time durations you want to sum up.
3. Press Enter to calculate the total duration.

The result will be displayed in the selected cell in the same format as the time durations you entered. You can also format the cell containing the total duration using the same custom format code [mm]:ss to ensure that it is displayed correctly.