I would like to modify select list for choosing menu parent for creating node (from the Menu tab). I am looking in
$form variable but I don't see this list data. How can I alter this select list?
Menu module does form alter to add the Menu Fields to the node creation form using function menu_form_node_form_alter in menu.module
Check your custom module weights to catch this data in your custom module to do form alter...
There is a new drupal 7 API called hook_module_implements_alter() which lets you alter the execution order for a given hook WIHOUT altering the module weights table.