The easiest way to do an air cut is to choose 3D offset in the fill-in sheet when loading a file. First, raise the z axis high enough to keep all depth of cuts above your material and choose 3D offset in the fill-in sheet. Where the tip of the tool is located is now considered X0 Y0 and Z0. When finished, this offset it canceled and the next time you run the file, the fill-in sheet will show no offset. Be sure you don't have the Z so high that it tops out with any safe Z heights (generally 1 or 2 inches to spare depending on your settings and/or file commands).
Scott
If guns kill people, I guess pencils misspell words, cars drive drunk and spoons make people fat.
"Those who hammer their guns into plows, will plow for those who do not" - Thomas Jefferson