Does the machine have a tool changer? Most do, so I'll describe the process, it's really easy.
After you turned it on, you did push the power up restart so the machine knows where it is, right?
Let's assume you will use will use T1. Push the MDI button. Push the T button, the number 1, then atc fwd button (or atc rev). Push the tool release button, and insert the tool into the spindle. Use the job button to just above the part, maybe use a 1/2" dowel as a feeler. Push the offset key. You will either see the tool offest page, or the work offset page. Pus a couple of times so you can see it switch, get to the tool page. Cursor to T1, push the "measure tool offset" button. This will record the current actual Z position in the T1 position. Then type "-.5" and push the write button lower right corner of buttons. This will set your T1 tool length offset.
Using the write button adds the value that you type to the offset array where the cursor currently is located. You can type in a negative or a positive number. This also works if you are touching off the side of a part, like allowing for extra stock or an edge finder, when you are on the work offset table page.