Insert Grid Button does not appear

Dear Support Community,

I downloaded Table JX 2.5.8 today and installed it. Installation finished successfully on Joomla 2.5.16 (up to date).
But no grid button does appear under the article editor window nor does the insert grid command work when adding it manually {insertgrid ID = 1} but I can create and save the grid in the JX editor without any problem.
I already tried to uninstall it, then changed all permissions to 777 and reinstalled it - but it's the same effect.
No button, no function on insertgrid command.
Any help would be appreciated!

Thanks and regards, Rubi