My 6 year old has only know my husband since she was 10 months old and we want to change her last name to his. The problem is we have no idea where my x husband is located. What can we do. I divorced my x-husband because of his use of drugs...

You need to get a family law attorney and start trying to locate your ex.

Like it or not, he can stop an adoption from going through--he's STILL your child's parent.

Why are you not collecting child support?

Yes, you need to start finding the ex,

Is there a child support order in place, a child custody order ?

Often if they have been forced to pay support, they are a lot more willing to sign over their rights if they know support stops if the step parent adopt

Now you really did not say adopt, but if you want to merely change the name without an adoption ( new husband still has no legal rights to child and child has no right to estate of new husband) but you still need ex's permission unless it was given at the divorce

Your husband will need to adopt the child. To do that you will need to get the father's permission or, at least, show a good faith effort to get it.

My x-husband keeps moving and when I left him we came to an agreement he would pay child support. Of course he never did and at the time my now husband had taken on the responsibility. I was going to place an ad in the last state I knew he was in and go from their. My x-husband is also very immature, just didn't want to put my baby through all the changes.

You need to consult an attorney to tell you the RIGHT way to go about this.