Enabling and Configuration
What is NotifyWhenAvailable?
NotifyWhenAvailable is an OpenCart extension that allows you to inform customers interested in out of stock products as soon as they become available. Thus, you can be sure that you do the maximum in order to avoid losing clients due to momentary stock emptiness.
Turning NotifyWhenAvailable On
In order to turn the extension on, you have to access its settings.
Accessing the Module's Settings
Log in to your admin panel. Then navigate to Extension > Modules and find NotifyWhenAvailable. Opposite of it, there is a blue Edit button.
Turning the Module On
Upon clicking on the button, you will access the module's settings - the Control Panel tab in particular. There, you will see two drop-down menus that look like this:

The first drop-down menu is by the name of Module settings and it serves as the on/off switch of your module. Set it to Enabled and click the Save Changes button in the top right corner of the page to turn the module on.
Assignment to Layouts
Basically, NotifyWhenAvailable's function is to produce a form (as a pop-up window) whenever a customer tries to add an out-of-stock product to his/her cart. Upon filling that form, the customer will be added to a record and will be notified as soon as the product is available. That said, you have to determine which pages you want the pop-up form to appear on.
Note: These pages should be either all or some of the pages that allow you to add products to cart.
To assign NotifyWhenAvailable to the desired layouts, navigate to System > Design > Layouts and choose the desired layout (Product is used for the example). Access the layout's settings by clicking on the blue Edit button.
In the newly opened page, you should see two tables. In the second table (the one below), click the blue + button titled Add Module and a row should be added to the table. From the drop down menu of the Module column, choose NotifyWhenAvailable, leave the other columns as they are and click the blue Save button in the top right corner.
Do this for as many layouts as you want.
Congratulations! You have sucessfully set up the NotifyWhenAvailable module.