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I got a used Marshall AVT150 head and a 1960A cab. I don't which input to plug the cable from the cab into the head. One input says 8#8486; and the other says 8/4#8486;. My cab says 16 ohms on the back. Can someone help? Thank you very much

Do you mean the the two outputs from the head are 8 ohm and 8/4 ohm and the cab has only one input with an impedence of 16 ohm? If that's the case, you can plug a speaker wire into the 8 ohm output of the head and the other end into the 16 ohm input of the cab.

that is what i did and it works fine but it doesn't sound louder than my MG100DFX and isn't it supposed to?

First of all, a higher wattage amp does not always mean it will be louder, but it will allow you to punch thru the mix more easily. The sound pressure level of the loudspeakers has more to do with the sound level than the amp's wattage. Second, 50 more watts will not make a lot of difference in sound level especially when you are using a 16 ohm 4x12 cab. The amp has to push 4 loudspeakers which will eat up that 50 watts in a hurry. Third, A guitarist usually plays about 10 feet in front of his amp with the loudspeakers of the amp below his/her ear level. The sound will usually shoot right by him/her before the sound waves really take shape. Try to listen to your amp about 30 feet away from it and you will notice how loud you amp can be. It will even sound a lot different from what you hear when you play your guitar just in front of the amp.