Please advise how to add freon to mitsubishi montero '01 air conditioner unit

The best thing i can tell you is to take it into a shop. You can add freon to a system but you need to know what kind your vehicle needs. If it takes the new current freon your in luck you can buy a recharge kit at any local parts store. If not you have a few options. You can get a conversion kit to change the system over to the new freon or you can try to find a shop that can recharge it with the older freon for your vehicle. Since its an 01 it should be the new freon but i cant be sure. There is a fitting with a cap on it on you AC Lines. should be 2 a red cap and a green cap. You remove the green cap and with the engine running attach a recharger bottle from the store and refill the system. Note the freon recharge kit will come with some instructions to always read them first. hope that helps good luck.