From Maspalomas Direct to Roque Nublo
The Line 18 bus from the main Maspalomas bus station just by the Faro goes up the Fataga Valley to San Bartolomé de Tirajana (Tunte) and then on to Ayacata village and Roque Nublo before finishing at Tejeda village.
Allow at least an hour and a half of travel time to get from the resort to the rock; We advise you to get the first bus that leaves Maspalomas at 06.25 as it gives you plenty of time to walk from the road up to the Roque Nublo platform, enjoy the view, and get back down in time for the bus back to the resort.
Note that this bus line is limited on Saturdays, Sundays and fiesta days and that some of the departures don't go all the way to Roque Nublo (those marked in yellow on the timetable only go to Tunte and those marked in grey only go to Ayacata). You can walk from Ayacata to Roque Nublo in under an hour but it's a steep trek up the road.
From Las Palmas to Roque Nublo
While the trip from Maspalomas to the rock is direct, getting to Roque Nublo from Las Palmas requires a change or two.
Start with Line 303 from the San Telmo bus station. This takes you to San Mateo hill town. They get the next Line 305 bus from San Mateo to Tejeda, then the Line 18 bus from Tejeda to Ayacata (tell the bus driver that you want to stop at La Goleta).
Alternatively, start walking at Tejeda (you are a steep hour's walk from Roque Nublo) or get a local taxi to the La Goleta car park.
We'd love to be more specific, but bus times and connections change all the time on this route. Basically, our advice is to get the earliest bus possible from San Telmo, take a windproof jacket for the waits, and treat it like an adventure. Oh, and take enough money for a taxi if you miss your bus.