I am a Canadian citizen and live in Canada.
I won some money in Las Vegas in 2008 and taxes were withheld; I was also issued a 1042 - s document, without a TIN.
How do I file with IRS to reclaim the taxes withheld?

You have no legal requirement to file because the amount withheld is set at the percentage owed by tax treaty, which, I believe, is 30%.

Now, if you can show that you LOST money in Las Vegas (easily done if you used the Player Cards provided by the casinos) in 2008, then you can claim those losses as an offsetting deduction and et a refund.

The form you file is Form 1040NR. The calculations are done on Page #4.

If you need professional help, contact me at [email protected].

Canadian Tax Treaty

Gambling winnings of Canadian residents are subject to NRA withholding at 30% on the gross proceeds from gambling won in the United States. However, Article XXII of the Canada - U.S. Income tax treaty entitles residents of Canada who are taxable by the United States on gains from wagering transactions to deduct U.S. source losses from U.S. source wagering transactions. Canadian residents should file Form 1040NR to obtain a refund of U.S. taxes withheld from the gambling winnings, if applicable.

So, you fill out your 1040NR tax return, deduct your gambling losses from your winnings to arrive at your taxable gambling gain (if any).

Then you complete a form W-7 following the directions carefully http://www.irs.gov/pub/irs-pdf/fw7.pdf
You will check box (a) and (h) and on line (h) write "exception 2. Read the instructions and requirements for that exception carefully. Now send the W-7 and all necessary documents and your 1040NR to:
Internal Revenue Service
ITIN Operation
P.O. Box 149342
Austin, TX 78714-9342