You have to first add conditions that allow the rule to know what fields are available.
To do this you can use combinations of the following conditions:
- Entity exists by property
- Entity has field
- Entity is new
- Entity is of bundle
- Entity is of type
Those conditions help rules identify the entity and bundle so it can show the relevant fields.
You can then access those field tokens in subsequent conditions and actions.
For example, you might use Entity is of bundle "Event" to then get access to fields used on the event content type.
In your case where you only have access to he user/profile entity you would have to add a condition like above to determine the entity/bundle of the user/profile so that you could get access to the field that references the event.
Then have an action to load the event entity (you could use the action "Fetch entity by id") using the entity id in that field.
Then you could add an if condition in the actions (using the conditional rules module) and say if the fetched entity is of type "Event" then if the value of your field is "Open" then do your final action.
It's a bit complex but you should be able to achieve what you want using the conditional rules module.
I think though that this will only work if you have a field on the user that references the event.
If it is the other way around and you have a field on the event that references the user then it would work differently.
In that case I would use the "Fetch entity by property" action to fetch content where the checkbox field = "Open".