You can also include the pagenav syntax in a template page in each of the namespaces you want it applied to:
1. Put this in main.php where you want it (likely after the wiki content):
<?php tpl_include_page('pagenav', true, true)?>
2. Make a page called "pagenav" inside each of the namespaces you want it applied to. Then add this code (for prev/up/next):
[<10>]
~~NOCACHE~~
Tada!
For a bit more clarification, see this post:
https://forum.dokuwiki.org/post/41858