Public transport timetables are regularly updated online both for the regional orange-coloured Volánbusz (scroll down and select 'Szentendre') and for the suburban rail line (pictured above) known as the HÉV (look for 'H5:' the Szentendre HÉV line and click one of the stations to download the pdf timetable in English and Hungarian). The latter link includes the tram, trolleybus, Metró and city bus timetables too.
The Volánbusz bus stop lies about 6-7 mins walk from REC on either side of Szentendre's dual carriageway called 'Road 11.' The stop is called 'Pismány ABC' (click to view the north and southbound bus shelters). Note that using these stops is cheaper than alighting/boarding at 'Danubius Szálló' which is closer to the REC.
Volánbusz not only journeys between Pismány ABC and Szentendre autóbusz-állomás (the town's central bus station), where you can change for the HÉV (note that for the HÉV you can buy tickets for yourself and your bike on-board too), but all the way to/from Budapest's Újpest-Városkapu autóbusz-végállomás in the XIII district (kerület), where in fact the ride terminates/begins, close to Arpad hid/bridge and from where you can pick up Metró 3, the so-called Blue line. Service numbers (shown on the route map) typically include 880 and 890 (and 883 and 893 on weekends and holidays).
Separate to the online timetables mentioned at the top of this page, a brand new (Hungarian-language) online journey planner for both Volánbusz and intercity rail can be found here, while further information on how to get to/from REC's Szentendre office is here.
REC offers a shuttle to/from the HÉV station at 0845 (at the exit of the road 11 underpass) and at 1700 from REC's front gate as a public transport 'feeder' service. Over 20 years it has played its part in supporting #inclusivemobility!