How do I open a 2006 Turbotax file?

You would have to open that file in the TurboTax 2006 program. 

The only way to open a 2006 TurboTax .tax file is with the CD or download TurboTax software for 2006. If all you need is a PDF or a printed copy, you could try TaxPrinter. You send them your .tax file and they send you back a PDF of the tax return, which you can then print with Acrobat Reader. TaxPrinter has no connection with TurboTax. I have never used TaxPrinter, so I cannot make a recommendation. I'm just making you aware of it.

@tessellon even if you had the cd to install 2006, updates are no longer available so you could possibly change the 2006 return from what was filed. also no state program is available