We can't load a collection by an id; a collection is multiple items where as an ID belongs to a single item. If you have the category and want to load it, use the following code:
$category = Mage::getModel('wordpress/post_category')->load($categoryId);
If you then want to display the category name you can use:
echo $category->getName();
Courtesy : Ben Tideswell (From Fishpig)