Chromium build 45.0.2416.0 (331982) is the last version which still has NPAPI plugin support enabled.
To get Flash plugin for Chromium you need the PPAPI plugin: PepperFlash.
PepperFlash is more secure because it works in sandboxed processes.
This plugin is a non-free (proprietary) software developed by Adobe.
Follow these steps:
- Install PepperFlash (PPAPI) from Adobe
Stable version - Direct link: install_flash_player_osx_ppapi.dmg