Menu: Adjust -> RGB Midtones...
The RGB Curves are non-symmetrical (same as Gamma curve). RGB Midtones have symmetrical characteristics.
The RGB Midtones is just a variation of the RGB Curves with different type of curve affecting more the middle tones.
Tip: If you want full control over the curves and their characteristic, we would recommend you to use the "Smart Curve" plugin distributed with PhotoBrush. It's a very nice freeware PhotoShop compatible plugin made by Alois Zingl. You can find and install this plugin via menu Filters >> Plug-In Manager. The Plugin is located right in Photobrush/PSPlugins folder. Don't forget to contact the author and let him know how nice this plugin is! ;)