It's a courtesy on B-Greek that we post the Greek for the verse in question, so I'll provide that here:
ὃς ἐν μορφῇ θεοῦ ὑπάρχων οὐχ ἁρπαγμὸν ἡγήσατο τὸ εἶναι ἴσα θεῷ.
Of course, the phrase in question is the last one: τὸ εἶναι ἴσα θεῷ. It's common for the neuter article to attach to an infinitive. It functions like a gerund in English, allowing the verb to behave like a noun (that is, serve as the subject of a verb, object of a verb, object of a preposition, etc.).
In this case, the verb ἡγήσατο takes two objects in the accusative, the one being held or regarded as something else. For example, you might say ἡγήσατο αὐτὸν βασιλέα, meaning that "he held him to be king."
"He did not consider (οὐχ... ἡγήσατο) being equal to God (τὸ εἶναι ἴσα θεῷ) robbery (ἁρπαγμόν)."
Some moralist might say: "I don't consider all lying a sin." A proud mother might say: "I consider my son a genius."
The question is what he ("Jesus Christ" from the previous verse) thought of being equal with God. Some people might think that equality with God is something to be grasped and sought after, but the text is saying that Jesus didn't think of it that way - and instead humbled himself, κτλ.*
Hope this helps.
* By the way, κτλ. is the Greek way of writing etc. It's short for καὶ τὰ λοιπά ("and the rest"), which corresponds to Latin et cetera
("and other things").
Jason A. Hare
Tel Aviv, Israel