I came across an extremely useful blog while reading Twitter today. Alex Anikiev has started posting a series of blog posts on importing data into AX 2012 using the Excel add-ins. He lists the steps necessary to fix issues as well as diagrams of the data models! Make sure to go through his blog archive.
I have not tried any of these but skimming through the articles they look very useful! Use any import information at your own risk!
The blog is located at http://ax2012exceldataimport.blogspot.com/, and currently has the following articles:
Generic Data Import Information and Setup Walkthrough
Importing Main Accounts
Importing Customer Transactions/Balances
Importing Vendor Transactions/Balances
Importing Ledger Transactions/Balances
Importing Inventory Transactions/Balances
Importing Sales Orders
Importing Free Text Invoices
Importing Purchase Orders