How do i download AI's for offline play?
If you can get online with the computer that you have OpenTTD, that is by far the easiest way to do it. Then just follow the guides that Brumi posted. (eg. use the online content download button in the main menu or AI settings dialog)
If you must use a different computer to download AIs and transfer them on a memory stick, you need to learn more details on where the AIs are stored in order for OpenTTD to find them. This is explained in the Readme of OpenTTD. You also need to download all AI dependencies manually. Eg. only use this way if you must.
After you have downloaded some AIs, you can disconnect from internet and play offline without any problem.
_________________My OpenTTD contributions
(AIs, Game Scripts, patches, OpenTTD Auto Updater, and some sprites)Junctioneer
(a traffic intersection simulator)