You are using the mysqli extension to connect and run your query, but then you use the mysql (notice the lack of i at the end) extension to get your inserted id, that can't work. While they are both extensions that provide access to mysql, they are also two very different libraries and can't share a connection between each other.
For the record, mysqli is the one you should be using, mysql is the "old" version that does not support new features of mysql >= 4.1
In other words, the solution is to use mysqli_insert_id()
Also, please escape your parameters properly, you can't put the content of $_GET and $_POST variables inside your query unsecured like that. At the very least use mysqli_real_escape_string()
$query = "INSERT INTO meetup.EventComments (EventID, OwnerID, Comment, DateTime) VALUES (" . mysqli_real_escape_string($eventID)." [...]
For more infos on this, have a look to the numerous questions on this subject, for example this one: How to properly escape a string via PHP and mysql