I was having the same problem. Plugin is activated but was not showing up in the publish box of an edit post page or in the quick edit section.
Did some debugging on the code and found that this line of code in the is_allowed function…
global $pagenow;
was returning an empty value so the plugin can’t work out what page it is on. Not sure why this would be the case, from what I have read it is most likely a conflict with some other plugin, which is the risk in using global variables. Anyway I changed this line of code to…
$pagenow = basename($_SERVER['REQUEST_URI'],'?' . $_SERVER['QUERY_STRING']);
which effectively gets the data from a more permanent location, I don’t know if this method will cause any other issues, but it seemed to solve the problem for me.
P.S. I also had an issue after this where everything displayed okay and you could change the post type, but when you clicked update it did not save the change. This turned out to be an issues with a another plugin, to fix it I changed post_type switchers add_action save_post priority from 999 to 11.
Good Luck.
