Thank you again for the help. I've finally managed to get it working. I've been following this
guide and waited for "it will automatically appear in the list of Recent Events". Finally it came to my "micro" mind to browse folders in the library and it was there. Installation passed without issues and it's working.
But still I wonder why it is not visible in pbsync app? There is one thing that bothers me and maybe it has smth. to do with the issue. There is an activation code from Bookland that is supposed to be entered on the device. But on the Inkpad I've just entered email and password and that's all it never asked for activation code?