Do not open the file on your computer as it will not work.
The file needs to be opened from your Android tablet so you will need to copy the file to that. Use the USB cable with the tablet, connect to your computer then enable the USB storage when it shows on the tablet screen. If it doesn't ask you to enable this, then tap the time on the tablet (bottom right) then tap USB Storage, then tap the button to enable.
Now on your computer you will have a new hard drive appear. This is your tablet. You can copy the file you downloaded over to the tablet just like copying a file from one drive to another. Once copied to the tablet, unplug the USB cable.
Open the File Expert application that is installed on the tablet.
at the top of the screen, tap "Folders". Then on the left side, tap "SD Card". You should now see the file you copied to the tablet on the right side. Tap the file and it will start the installation off.
If it asks you to enable installation then you need to do the following:
On the left side, tap Security
Tap "Unknown Sources"
Now press the home button, open File Expert again then tap the com.bagpuss file to install the Play Store installer.
Once it has been installed, press the home button. Press the button to show your applications, then tap Play Store Installer. Follow the prompts to install.