I'll help you. Maybe you have an answer, maybe you don't.
The word in question is ἡμῖν which means "us" unless you have a different definition. The verse in Greek says,
γνωρίσας ἡμῖν τὸ μυστήριον τοῦ θελήματος αὐτοῦ, κατὰ τὴν εὐδοκίαν αὐτοῦ ἣν προέθετο ἐν αὐτῷ
The question, then, is who is the "us" in this verse (Ephesians 1:9)? Is it Paul included with the Ephesians? Then why does it say,
Ephesians 1:12
"so that we who had already put our hope in Christ might bring praise to his glory." and this is contrasted with,
Ephesians 1:13
"In him you also were sealed with the promised Holy Spirit when you heard the word of truth, the gospel of your salvation, and when you believed."?