I had the same error as Matthew and after trying this solution I could see that Saran's fix had already been implemented into the "npm pi-pgio".
If you're still having this problem this solution worked for me.
You need to update the path in gpio-admin.
First Download gpio-admin and open source code (I used nano, but any editor will do)
$git clone https://github.com/quick2wire/quick2wire-gpio-admin
Check this file and make sure that the paths are updated. Update the
function allow_access_by_user() the path needs to be updated to
Now navigate into the folder and install
$sudo make install
Note: you may need to add yourself as a user
$sudo add adduser $USERNAME gpio
Hope this helps save some headaches!