1st you need to get a gravity reading. Then with the help of WineCalc you will know how much sugar to add.
NEVER add sugar blindly or add acid blen with out getting a hydrometer reading and doing a TA test for acid.
Concord and Muscaidne wines can be handled two ways, either they can be made as all graoe wines and deal with the higher acidity with either cold stabilizayion or by the use of additives like Pot. carbonate or by diluting with water to get the acidity down and then add suagr back to get the needed sg.