Forward/Direct synctex (source to pdf) can either be activated by selecting 'Navigate, select, and edit' > 'SyncTeX from cursor' in the side bar, or by the shortcut ctrl+alt+j (cmd+option+j on Mac), see also the FAQ for an alternative shortcut. Backward/Reverse synctex (pdf to source) is activated by pointing at the relevant element of the pdf preview. When using the internal viewer, the default keybinding to point at an element in the pdf preview is ctrl+click.