Step One: Click on "Edit an existing mod" in the "Modding" section. If you're on the Dashboard you can simply click on "Open mod file" on the "Open a mod" tile.
Step Two: Select the mod you want to open and click on "Open".
Step Three: You're done! The mod should have been loaded correctly and will be converted automatically if you save the mod.
Alternative: When you are on the dashboard, you can drag & drop the file into the "Open a mod" tile to open it.