Firstly, you have to clean that area... if you have alcohol, that would be best, or with any anti-septic cream or lotion.
Secondly, don't use the concealer on the cut skin... it mightn't do much good. Once the wound has healed, you can start smearing tea tree oil lotion (there are quite nice scar removal stuffs at Boots- the more pricey, the more effective), and just wait for the scar to efface... might take some time. Once it is just a scar, you can start applying the concealer.