You can add the salt now. However, have your water tested for everything first, including salt.

Test for:
Free Chlorine
Total Chlorine
pH
Total Alkalinity
Calcium Hardness
Cyanuric Acid (also called conditioner or stabilizer)
Salinity - IMPORTANT because if you add too much salt, you will have to drain down the pool to dilute the salt level.

Your water should be adjusted so that it is balanced, according to the saturation index. Your pool store should know what that is. If not, find another store. And as everyone here will tell you, get yourself a good test kit and learn how to test your own water.