If you opened the app and clicked on minecraft, click on the button "create Custom Profile"

Then is there a little window.

This Window should open. You can give the profile any name. Write the name in the first row.

In the next row you can choose your minecraft version, down below is the forge version.

My Mod is for the 1.15.2 with the forge version forge-31.2.0

All my mods are for this version!!

If you done with the options, there should be something like this in the picture. Click on it

If you clicked on the profile this will show up. Click

"Add More Content"

If you clicked "Add More Content" this will open. Now you can search something in the search bar.

I searched as example my own mod:

Subnautica Below Zero

If you find your mod click on install and the newest version will be installed. Some mods will not be showen because its the wrong version then you have to make another profile with another version.