Hi, i'm gonna give my process that solved this problem on El Capitan with my Late 2008 MacBook Pro and an Intros Pro Medium.
It has been quite a long investigation, but now it's fixed and i think it should work for most of you.
One last thing before beginning: if i make some mistakes in english, it's because i'm french :-) but i'll do my best to be understandable...
So, Wacom and El Capitan troubles: it's still about modify the permissions, but a new security system in El Capitan prevent from any change and the Disk Utility has become really useless... By the way, this is a shame.
Well, let's go:
ReBoot in Recovery Mode by rebooting and pressing « cmd R »
Once in recovery mode, open a Terminal window from the upper menu and write
csrutil disable
This will disable the new feature made in Apple and arrived with El Capitan: it’s called SIP (System Integrity Protection) and it’s necessary to disable it if you want to play with the plist files…
If you prefer to leave SIP enabled while disabling the filesystem management: you can type « csrutil enable —without fs » this will allow you yo change permissions in the filesystem and keep SIP running.
When it’s done, reboot (yeah, now we begin to feel like on a microsoft os…)
Open a Terminal window and type:
f=/Library/LaunchAgents/com.wacom.wacomtablet.plist;sudo defaults write $f KeepAlive -bool true;sudo chmod 644 $f;launchctl load $f
Assuming that the com.wacom.wacomtablet.plist is in Macintosh HD/library/LaunchAgents
This command line will modify the permissions on the plist and will re-enable it.
Then it should load properly.
If you still have a message such as « Service cannot load in requested session »
Or a message like « Service is disabled »
You may try to write in a Terminal window:
launchctl load -w /Library/LaunchAgents/com.wacom.wacomtablet.plist
Still assuming that the com.wacom.wacomtablet.plist is in Macintosh HD/library/LaunchAgents
This command line will overrides the disabled key and set it to false. This is what the -w do.
Now, if you want to put the SIP back on, you can reboot in recovery mode, open an new Terminal window and type:
csrutil enable
And reboot again…
If you have a much cleaner approach, please, feel free to share it.