After downloading the extension, perform the following:

1. Open any project.

2. Click Settings -> Extensions -> Install Extension.

3. Select the Mathematics Development Engine Extension.

Once an extension has been installed, it will be available for any project. However, it will need to be enabled on each project. To enagle an extension, follow the following steps:

1. Open your project.

2. Click Settings -> Extensions.

3. The Mathematics Development extension should be visible in the list. Click "Enable".

4. Save and re-open your project.

Warning: The Mathematics Development Engine Extension should be enabled before any behaviors are imported! Otherwise, you may see errors about "missing blocks".

Problems? Up to date engine extension installation instructions can be found at the official Stencyl website.