You manage your Add-ons from the Add-Ons tab.
This can be accessed by pressing Setup, Config, then Add-ons

The green check mark next to the Add-on pack, indicates that the Add-on is activated. (See above)

A lock symbol indicates the Add-on pack requires activation. (See below)

You can purchase the dPack for your Avantis or Avantis Solo from the online shop.

https://www.allen-heath.com/product/avantis-dpack/

To use your add-ons, press the Library button, on the processing tab relevant to the Add-on (for Compressors, this would be the Compressor tab, for Preamps, this would be the Preamp tab) and select your Add-On from the list. Then press Recall.