Download tweener-install.py and drag the file into Maya's viewport. The automatic install will also ask to create a shelf button for you.

You can execute the following Python commands to load the plug-in and open the tool window

import maya.cmds as cmds

if cmds.pluginInfo('tweener.py', query=True, registered=True):
    if cmds.pluginInfo('tweener.py', query=True, loaded=False):
        cmds.loadPlugin('tweener.py')
    cmds.tweener()
else:
    cmds.warning('tweener.py is not registered')

The automatic installation may fail due to firewall permissions, in which case you can use the Offline Installation instead by downloading the latest release and locate the ZIP file when prompted. If that fails as well you can try the manual installation as a last resort.

Download the latest release from the Releases page.

Unzip the file and copy the contents to a folder named tweener inside your Maya's plug-ins folder.

Typical locations for the plug-ins folder:

Windows: C:\Users\<username>\Documents\maya\plug-ins\

macOS: ~/Library/Preferences/Autodesk/maya/plug-ins/

In some cases the plug-ins folder will not exists, in which case you can just create it.

Go to the modules folder inside your Maya preferences directory. Again, if it doesn't exist you need to create it.

Create a new file and name it tweener.mod inside the modules folder, and add the following lines where you replace the path to your own plug-ins directory:

Windows example:

+ Tweener 0.0 C:\Users\<username>\Documents\maya\plug-ins\tweener
MAYA_PLUG_IN_PATH +:= 

macOS example:

+ Tweener 0.0 /Users/username/Library/Preferences/Autodesk/maya/plug-ins/tweener
MAYA_PLUG_IN_PATH +:= 

You need to restart Maya after copying the files. You can verify the installation of the plug-in by seeing if tweener.py shows up in the Plug-in Manager. All plug-in paths can be found using the following Python code in the Script Editor:

import maya.mel

paths = maya.mel.eval('getenv MAYA_PLUG_IN_PATH').split(':')
for p in paths:
    print
    p

After the plug-in is succesfully installed, execute the following lines as Python or add them to a shelf button:

import maya.cmds as cmds

if cmds.pluginInfo('tweener.py', q=True, registered=True):
    if cmds.pluginInfo('tweener.py', q=True, loaded=False):
        cmds.loadPlugin('tweener.py')
    cmds.tweener()
else:
    cmds.warning('tweener.py is not registered')

The icon is located in the icons folder where you installed the plug-in.