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This is more or less how Rav Chaim Volozhiner explains the Mishna (as I remember it):
Obviously, we are required to serve Hashem whether we like it or not. As our Creator and Master, He has total rights over what we do, and we must obey.
If a person can not motivate himself to serve Hashem without thought of reward and punishment, then he should serve based on that motivation (????? ????? ??? ????).
This is not someone who can rightfully be called an eved at all. Hopefully, be’ezras Hashem, he will eventually reach the level of serving simply because it is what Hashem “wants” (????? ??? ???? ?? ????).
On a deeper level, one may argue that Hashem “wants” to reward us, so serving with thought of reward should qualify as lishma.
In fact, this is true, but only if the thought of reward is purely intended to satisfy Retzono Yisbarach. For most people, this is not possible.
There were some “Avadim” (slaves who served because it satisfied their Master’s Will) who were able to incorporate thought of reward into their service in this way. Outstanding individuals such as Moshe Rabbeinu, Avraham Avinu and David Hamelech were ????? ??????? ?? ??? ?? ??? ???? ???.
The Mishna cautions us not to try to emulate their example, since for most people it would be counterproductive. Of course, we should strive to be Avadim (slaves, who serve not out of any thought of personal gain, but because they must obey their Master), but we should be content with the status of ????? ??????? ?? ??? ??? ?? ??? ???? ???, which is the lower of the two.