Congrats, glad it got there OK. The middle section should slip easily into the charcoal section. You should be able to rotate the middle section within the charcoal section without it binding; if it binds, then one or both of the sections is a bit out of round. If this is the case, read the post just below, "New WSM Fit" for details on the fix.
The access door should sit flush against the middle section. I like to adjust the curvature a bit so it has a slight spring-like action, so I have to push on the knob slightly to latch it, giving me a good fit against the middle section. Just bend by hand, making small adjustments, til you achieve a good fit.
Let us know how things go this weekend!