If you land at Geneva International Airport or if you travel by train from within Europe via Geneva, you can reach Locarno by train.
There are connections every 30 to 60 minutes both from Geneva Airport and Geneva railway station. The trip takes between 5 and 6 hours via various routes: Simplon-Domodossola (through Italy), Olten, Luzern, Zürich.
Advance booking of train tickets within Switzerland is possible but not obligatory.
On the SBB webpage you can find the most convenient train schedule for you.
Once in Locarno, continue to Monte Verità by following the directions given.