You cannot claim the exemption on your tax return. The exempt amount is not supposed to be included in the wages on your W-2. The W-2 should show only the amount you were paid above the $600 exemption.
Check with whatever department issued your W-2 about how they calculated the amount of wages on the W-2. They should have already subtracted the exempt amount. If they did not, they might have determined that for some reason the payments that you received do not qualify for the exemption. If your W-2 is incorrect, ask them to issue a corrected W-2.
The exemption is actually a maximum of $50 per month, so $600 is the maximum for the year if you were active all year. If you were not a volunteer for the entire year you would not be entitled to the full $600 exemption.
