Well, your in luck because you can do a lot with events. You can make countries declare independence, you can make a country's ideology change, you can annex provinces, and so much more! The menu is easy to use, so I don't know why your having so much trouble figuring out how to use it. Never the less, heres how you do it.
Step 1: Go to the events menu
Step 2: Select "Add New Event"
Step 3: Title the event (the title can be what ever you want".
Step 4: Click the "Recipient" menu and select what country you want to receive the event.
Step 5: Select what date you want the event to occur by clicking "No Date". You can also choose when you want an event to end, but this is optional.
Step 6: Click "Add new trigger and after that click "Add New Condition". You'll then be taken to a menu were you can select what condition you want to have in order for the event to occur.
Step 7: Click "Add new outcome" and you'll be taken to a new menu where you can title your outcome and edit the chance of the outcome occurring. Then click "Add new outcome" and select what you want to happen in the outcome.Once your done, make sure to save. If you want multiple choices to be available, then just add another outcome. If you want more than one change to occur in the same outcome, just add a new occurrence in the "outcome" menu.
Step 8: If you want to make your event happen multiple times, click the "repeatable" setting.
Step 9: If you want your event to have a picture, then click the "pop-up" option. You will have to add the picture manually by putting it in the game files. To access the game files, go to your C Drive and go to the files Steam > Steamapps > common > Age of Civilizations II. Alternatively, you can go to the AoC2 steam menu, right click play and select "Access game files". Once your in the game files, go to UI > Events and in the events folder select the image you want to use on your computer and put it in their. Then exit out and start the game up again. Once your back at the event menu, click on the "Picture:" icon and change the name to the name of the image you want to use. Thats it.
Once your done customizing your event, click "save event" and after that click "save" on the main event menu. And bam, now you (hopefully) know how to add custom events.
If you want to do a certain thing with events, just ask me and I can tell you how.