What are the connotations of the Pasuk "u'Shemartem es Kol Chukosai ... va'Asisem osam"?
Seforno: If you will study Hashem's Chukim and Mishpatim and learn how correct they are, you will go on to fulfill them.
Moshav Zekenim: Go in Hashem's ways - He judges fairly
Why does the Torah add "Ani Hashem" here?
Seforno: To teach us that one should take care not to add to the Mitzvos or subtract from them, since Hashem's deeds and His Mitzvos are perfect and are therefore not subject to addition 1 or subtraction.
He refers to adding or subtracting and considering this to be the Torah Mitzvah. Chachamim are instructed to add, to make fences for Torah - "Asu Mishmeres l'Mishmarti" (Mo'ed Katan 5a)! (PF)