This song title seems to be a play on words - a kind of joke based on a word that has several meanings. It is common for song writers to do this, as it makes the song more catchy and memorable. If a person is 'officially missing', it means that the police have filed a report on someone who has disappeared. If a person has not been seen for several days or weeks, their status is 'officially missing' and the police might carry out searches for them. 'I'm missing you' means that I'm sad or lonely because you're not with me. To say that 'I'm officially missing you' references the other meaning of 'missing' described above. In terms of the message of the song, it means that I'm missing you a lot. Note that this isn't a standard usage, though - it's just an appealing play on words, possibly invented by the songwriter.

Maybe the government secret service is on the lookout for a missing person? Or it could be just that you are missing the person and want to make it loud and clear by making it official.

To add to what Darryl Wee has said, consider the following. It means that now I'm missing you so much that there is no confusion about whether it is true or not. Compare that to this situation. Some people at the office are saying that everyone is going to get a bonus. At this point, you are not sure whether this is true or not since you haven't received an e-mail from management. Once your boss announces the bonus at a meeting, it's official. You can then say that you were officially told that you would receive a bonus.