When you upload a project CircuitHub will attempt to automatically reconcile each BOM line. 

We recommend checking out the following article for the best practices to get your project to auto-reconcile:
http://docs.circuithub.com/managing-your-projects/how-should-i-specify-part-numbers-in-my-eda-tool

If we are unable to auto-reconcile a BOM line you will need to manually reconcile it by approving a part number. CircuitHub will attempt to suggest parts based on the attributes for the line. If no correct part is suggested then you can search for it using the Octopart search bar.

If you are still unable to find the correct part please see the following on how to add a part manually:
http://docs.circuithub.com/managing-your-projects/how-should-i-deal-with-parts-which-are-out-of-stock-or-not-available-on-circuithub