X-Cart 5.5.x allows installing add-ons only from X-Cart Marketplace. To install add-ons, you must have a registered X-Cart installation.
To start add-ons installation:
Open the My Apps page in your store's Admin area (Apps > My Apps) and click the "Go to external App Store" link.
X-Cart Marketplace will open.
Search for the add-on you need to install.
For add-ons included in your software subscription:
Click the Get add-on button.
In the popup, click Install.
For add-ons not included in the subscription:
Click Add to cart and follow the steps to buy the add-on.
After the purchase, you will see a popup confirming the payment was successful. Click the Activate now button.
NOTE: If you select to install the add-on later, it will be added to the mini cart widget on the My Apps page in your store's Admin area and installed when you click Apply changes.
Your online store will start the add-on installation process with redeployment.
When the redeployment completes, you will see a screen as follows.