How to Install a Minecraft Modpack

In the web-panel, there's a side panel on the left of your screen. Under the MINECRAFT section click on MODPACKS.

Steps

In the MODPACKS page you'll see the custom button in the top right (colored in cyan/aqua).

By clicking on it, a small menu pops up; Go to the File upload section and upload your modpack.

The last step is to restart the server.

FAQ:

Do I have to change the server's egg?

No you do not, the Modpack Installer does it automatically for you.