Where can I add xposed modules to systemless xposed framework? I have installed Magisk and systemless xposed. What do I need to do to download and install xposed modules (not Magisk modules)? 

I don't see any app for xposed only the Magisk Manager from where I can install Magisk modules (like systemless xposed).
From Magisk tap downloads and search for Xposed


Choose Xposed version corresponding to your Android version- choose the version by rovo89

Tap on the module, on the next screen it will prompt you to download an installer. Do that, give permissions for installing for external sources when prompted. Go back to the earlier screen and install. Let it do it's job and reboot.

You would get the familiar Xposed installer. You are now good to install systemless Xposed modules