This is how he explains יפה שעה אחת בתשובה ומעשים טובים בעולם הזה - literally: One hour of teshuvah and good deeds in this world is better [than the enjoyment of the world to come.] He says it means that any moment when we bring unity between HaShem and His People (through Teshuvah and mitzwoth) is beautiful to HaShem.
This is a unity that is uniquely applicable to this world. As the Ba'al HaTanya explains that this world allows our souls to experience HaShem thoroughly, on a higher level [ie. more directly] than all the experiences of the upper worlds.