"Contents" menu on the left side.
The second line is "Tools" dubleclick.
Red Plus, then add the tool.
Now you created a tool globaly for all the devices...
"Contents" menu again, select Devices. Then Select Types tab. You can create a custom device type (computers), the double click it. There will be a tool tab, where you can specifi custom tools for these device types. Now you have only to do: change the computer's types (on the device) to computers.
I hope this helped