To import demo data into AX 2012 R3 version, Test Data Transfer Tool was put in place.
The Test Data Transfer Tool (beta) has been moved. It is now available from the Downloadable tools section of Microsoft Dynamics Lifecycle Services.
Download the test data transfer tool from the below link;
https://lcs.dynamics.com/v2/LocalTools/
Steps to import data are well described here
Test Data Transfer Tool – Video
Microsoft Dynamics AX 2012 Test Data Transfer Tool (Beta) Updated 7th Jan 2014 Video