What does "Your facts are useful, and yet they are not my dwelling, I but enter by them to an area of my dwelling" mean?
The line represents what Whitman feels about the branches of study that rely heavily upon facts and data driven reality to construct consciousness. Whitman precedes the line with discussion of "the geologist," "the mathematician," "positive science" and "the chemist." All of these professions rely heavily upon facts. For these disciplines, facts are "useful." In a sense, factual data is the "dwelling" for these individuals. Each profession requires factual understanding of reality as a part of its construction.
It is with this distinction that Whitman draws a different reference point between how he constructs reality. He does not reject "facts," for he understands that they have a role in consciousness. Yet, Whitman speaks to a condition that embraces a larger construction of reality. He is striving for something more permanent and less driven by data based reality: “Apart from the pulling and hauling stands what I am, . . . Both in and out of the game and watching and wondering at it.” For Whitman, the data of the professions he sees are what can allow him to "enter" into an understanding at what "it" is." Yet, Whitman wants to know what "it" is and he feels that factual understanding is not going to move him into this dwelling. Rather, it is going to be something larger, of which factual understanding might have a role. Yet, in seeking to understand and appropriate a larger life force, facts will not enable him to embrace the "dwelling" he so seeks and wishes to achieve.