Element Actions for Entries being viewed from modifyEntrySources
The checkboxes will only show up if there is at least one available element action for the current source.

Element actions are defined by element types’ getAvailableActions() methods, which will vary their response based on the $source that is passed in.

In EntryElementType’s case, if an unrecognized $source is passed in, then no actions will be returned.

However, EntryElementType::getAvailableActions() does have a hook called addEntryActions, which gives plugins the opportunity to add additional actions to its response.

So on top of using the modifyEntrySources hook, you can also use the addEntryActions hook to provide the actions that your custom source should have.

public function addEntryActions($source)
{
    if ($source == 'myCustomSourceKey')
    {
        $elementType = craft()-&gt;elements-&gt;getElementType(ElementType::Entry);
        return $elementType-&gt;getAvailableActions('section:X');
    }
}


(Replace X with the ID of a section you want to mimic.)