How do I install the Flash debug player in Chrome?
Disable the bundled plugin on about:plugins and then install the debug version.
EDIT: be sure to click on "[+] Details", and only disable the version of Flash that is based on the Chrome directory (the 'built-in' one). (Thanks @zeh for that additional hint in your comment)
- Open the Google Chrome plugin settings (enter "chrome://plugins/" in the URL and hit enter) and find the Flash player plugin. Click on the details link to expand the plugin details.
Plugin Screen 
!
Plugin Screen showing Details 
2A. If it says "Flash (2 files)" then check the location property of each entry and disable the one that resides in the Applications directory/Google Chrome/... directory (usually the first entry) by clicking the Disable link button. This is the disable button next to the "MIME Types" value not the Disable button aligned to the far left of the page (which disables both plugins).
2B. If there is only one entry, ie it doesn't say "Flash (2 files)", then download and install the newest Flash Player content debugger version here. The Flash Player installer will require you to close any browsers before installation can complete. After installation is complete restart Chrome and go to step 1.
Your plugin screen should look similar to the Plugin Screen showing Details image shown above.
As of April 2015 the NPAPI plugins won't work in Chrome. And for some reason I was not able to get the Adobe "Install Adobe Pepper Flash Player Debugger.app" installer to correctly update Chrome on OSX. The installer would run but would not update Chrome.
If you find yourself in this situation you might try the following...
My hacky solution (after much frustration) was just to end-run around the installer and chrome to manually copy the installed plugin from where the installer was putting it, into the place chrome was expecting it.
The installer was placing here:
/Library/Internet Plug-Ins/PepperFlashPlayer/PepperFlashPlayer.pluginChrome was looking for it here:
/Applications/Google Chrome.app/Contents/Versions/42.0.2311.90/Google Chrome Framework.framework/Internet Plug-Ins/PepperFlash/PepperFlashPlayer.pluginAll I did was replace the bundled installed non-debug version of pepper flash with the one debug version installed by the player. This seemed to fix the problem.
YMMV