By default, menus in IMP and in Horde in general, are collapsed. They have a little (+) plus sign next to them which you click to expand the menu.
This explains how to make menus expanded by default.
Find horde/imp/lib/Block/tree_folders.php and look for the $tree->addNode entry that affects the menu that you want to change.
Under a Debian Sarge installation the file is located under /usr/share/horde3/imp/lib/Block/tree_folders.php.
In line 159 - 161: the expand option defaults to false change it to true (see example below):
$tree->addNode($parent, $registry->get('menu_parent', $parent), sprintf('<b>%s</b> (%s)', $registry->get('name', $parent), $unseen), $indent - 1, true, $node_params);