How would I make the adjustments I could make with Curves in PS?
If you did not use different adjustments for each color channel, the solution is to use a black to white gradient map adjustment layer in luminosity mode. The stops in the gradient map play the same role as the points in the curve. As a matter of fact, it's simply a different visual interface, but the map or the curve are strictly equivalent.
If you are using Windows, I recommend using the 'smartcurve' plugin. Very powerful and useful for Elements, since it can work in 8 or 16 bits (with HSL or Lab additionnally), and create and use curves files used in Photoshop.