When I connect the electricity on the timer after less then one minute I hear A little explosion and when I looked to the board I saw a blister on the capacitor.
I have maybe the electrolic capacitor not good mounted. 
The image in the manual is not clear at all. What posititief or negatief is not two see on the picture.
Even when I use microscoop to see what positief is. I saw a + so I have connect the smallest pin for positief.
Is this correct? 
No, this is not correct.
The longest pin is the positive and corresponds with the ‘+’ sign on the PCB
The negative (shortest pin) is marked with ‘-’ signs on the enclosure of the cap.
You will have to replace it.
Strange because the board work.
Will I have to replace them both?
Can I test the capacitors with multi-meter?
The explosing I thought it was of a to high voltage. Because on the power supply stands 12V and when I use the multi meter it was 16V.
When I use another power supply the board worked. 
You will have to replace them.
You cannot test them with a multimeter.
It might seem to work but it is more reliable with the cap in place.