Hi,
Unfortunately, pagination within message boards is not supported by default, however you can still add the 'UniPager' control to '~\CMSModules\MessageBoards\Controls\MessageBoard.ascx' file and link it to <cms:QueryRepeater ID="rptBoardMessages" /> control, in this way you should be able to use paging. Please see following example code for unipager control:
<cms:UniPager ID="up1" PagerMode="PostBack" PageControl="rptBoardMessages" PageSize="2" runat="server">
<pagenumberstemplate>
<a href="<%# Eval("PageUrl") %>"> <%# Eval("page") %> </a>
</pagenumberstemplate>
</cms:UniPager>
Please note that you can add additional pager templates according your needs.
P.S. Following knowledge base article could be useful for you:
Ajax functionality for UniPager connected with RepeaterBest regards,
Ivana Tomanickova