Here is the debunking of it.
It is true that Kerry missed a vote to authorize defense spending and thereby "fund our troops in combat." In fact, he missed two votes. He missed one vote on an amendment to authorize an extra $25 billion as a "contingent emergency reserve fund" for possible use to support operations in Iraq and Afghanistan. Kerry also missed the vote on the overall $420 billion Pentagon authorization bill.
However, Kerry’s absence had no effect on the outcome of the vote. The larger authorization bill (S. 2400) passed 97-0 on June 23rd. Kerry was one of three senators who missed the vote. The other two were Republicans, funny how the Bush ad failed to mention that. Kerry was one of five senators who missed the 95-0 vote on June 2 to authorize the extra $25 billion "reserve fund." The missing five included two Republicans. One vote passed 97-0 and the other passed 95-0 so John Kerry's vote was not needed. President Bush knows this, yet he still runs these deceptive and misleading ads meant to fool the American people.
http://factcheck.org/article.aspx?docID=215