Page.EnableViewState = true;
at the beginning of your page load event
By default in mojoPortal content system pages have ViewState disabled to make the page lighter weight (less text rendered) which improves performance. When needed in a module control you can override the default with the line of code I suggested.
If you don't like the default behavior, you can change it in Web.config, look for this:
<add key="DisablePageViewStateByDefault" value="true" />
and set it to false. I recommend leaving it and just use the line of code where needed.
Hope it helps,