You'll find it a 10-15 minute walk from Bohaterów Monte Cassino, though it is a steady uphill route all the way there. Walk up Monte Cassino, passing under the railway line and the main road before carrying on along ul. 1-go Maja to the end. Next, there is a pathway going up with Hotel Opera on your left, which will bring you to the main gate. Allow yourself at least 30 minutes to get to your seat from Bohaterów Monte Cassino on a busy night.
The city's Forest Opera can be contacted directly or refer to BART in regards to events and concerts. Even when there isn’t a performance, this legendary venue is open for visitors who want to tour the grounds and exhibits (unless there's snow, check their FB page for availability). Also, be aware that the site listed here is the official site of the Forest Opera as there are a few unofficial ones out there!