Make sure the block that you’re working is of the type catalog/navigation. If you’re editing catalog/navigation/left.phtml then you should be okay.
<div id="leftnav">
<?php $helper = $this->helper('catalog/category') ?>
<?php $categories = $this->getStoreCategories() ?>
<?php if (count($categories) > 0): ?>
<ul id="leftnav-tree" class="level0">
<?php foreach($categories as $category): ?>
<li class="level0<?php if ($this->isCategoryActive($category)): ?> active<?php endif; ?>">
<a href="<?php echo $helper->getCategoryUrl($category) ?>"><span><?php echo $this->escapeHtml($category->getName()) ?></span></a>
<?php if ($this->isCategoryActive($category)): ?>
<?php $subcategories = $category->getChildren() ?>
<?php if (count($subcategories) > 0): ?>
<ul id="leftnav-tree-<?php echo $category->getId() ?>" class="level1">
<?php foreach($subcategories as $subcategory): ?>
<li class="level1<?php if ($this->isCategoryActive($subcategory)): ?> active<?php endif; ?>">
<a href="<?php echo $helper->getCategoryUrl($subcategory) ?>"><?php echo $this->escapeHtml(trim($subcategory->getName(), '- ')) ?></a>
</li>
<?php endforeach; ?>
</ul>
<script type="text/javascript">decorateList('leftnav-tree-<?php echo $category->getId() ?>', 'recursive')</script>
<?php endif; ?>
<?php endif; ?>
</li>
<?php endforeach; ?>
</ul>
<script type="text/javascript">decorateList('leftnav-tree', 'recursive')</script>
<?php endif; ?>
</div>