International Bus Station is at Meerssenerweg behind the train station. Cross the station to reach the stop. Monday-Friday the station is opened 0545-2300, Saturday 0600-2300, Sunday 0700-2300. Outside these hours, walk around the station. Navigation can be found here:
https://bit.ly/maastrichtflix
The station is at the new bus terminal in front of the S-Bahn station Leverkusen-Mitte.
FlixBus will depart from the bus platforms in the east of the terminal.