It is recommended that volunteer training NOT be done using the IRS Practice Lab. Most of the links at the IRS Link & Learn are broken and you will not be able to get there. Any ID or password you may have had from last year will probably not work.
Instead contact your site or district coordinator and get a login for TaxWise Online (TWO). Follow the instructions in the Pub 4012 Spiral (the "blue tab" page 7-1).
For training in December 2012 and January 2013, everyone should be using: