Hello @c_hm,
Thank you very much for using the theme.
The link that you have posted is of the documentation of the pro version and the option to set the slider using post, images, category is only available in the pro version of the theme but not in the free version.
The free version only has the option to set the slider using the pages as you have mentioned.
So if you want to add the additional option for setting the slider then you can always consider buying the pro version.
Not only this pro version comes with lots of other exciting feature and to know about it please refer to below given link.
http://themepalace.com/instructions/themes/business-field-pro/
Thank you.
Best Regards!!