They are not Previous / Next buttons in the same way as a web browser. The buttons are for the previous and next topics in a browse sequence and if the topic displayed is not in a browse sequence, the buttons are disabled.
First question therefore is have you created a browse sequence or are you looking for buttons that function the same way as a browser, in other words they scroll through the topics the user has viewed.
If the latter, there are many posts with how to achieve that but post back if stuck.
See www.grainge.org for RoboHelp and Authoring tips

@petergrainge
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