That package is downloaded from AEM Package Share - not a link. Instructions on how to download it are provided in the AEM docs:
GETTING STARTED WITH THE SAMPLES
To follow the tutorials on this page, install the package called Using ExtJS Widgets in a local AEM instance and create a sample page in which the components will be included. To do so:
In your AEM instance download the package called Using ExtJS Widgets (v01) from Package Share and install the package. It creates the project extjstraining below /apps in the repository.
Include the client library containing the scripts (js) and the stylesheet (css) in the head tag of the geometrixx page jsp, as you will include the sample components in a new page of the Geometrixx branch: in CRXDE Lite open the file /apps/geometrixx/components/page/headlibs.jsp and add thecq.extjstraining category to the existing <cq:includeClientLib> tag as follows: %><cq:includeClientLib categories="apps.geometrixx-main, cq.extjstraining"/><%
Create a new page in the Geometrixx branch below /content/geometrixx/en/products and call it Using ExtJS Widgets.
Go in design mode and add all the components of the group called Using ExtJS Widgets to the design of Geometrixx
Go back in edit mode: the components of the group Using ExtJS Widgets are available in the Sidekick.