First put this in the _root in the timeline. change 'mc' to the name of your mc that you are wanting to tab in.
Now inside of your mc, put this in the timeline. Be sure to change 'txt1' and 'txt2' to the instance name of your text fields you wish to tab between:
txt1.tabIndex = 1;
txt2.tabIndex = 2;
To tab between buttons put this but change 'btn' to the name of your button.:
btn.tabIndex = 3;