I had this exact same problem and figured out a way to view your reviews after you've already accepted :
When you have logged on to the Fastlane web site, follow this link:
https://www.fastlane.nsf.gov/grfp/ViewApplReview.do?method=loadApplRev
FYI, I had first contacted the Fastlane helpdesk and then the GRFP program office about the problem (directions below) but didn't get a response.
"We have received the following information from the GRFP Program Office. To receive access to your reviews, please follow the steps below.
The GRFP will disapprove pending tenure requests to allow new Fellows to see their reviews. To request this, please send an email to grfp@nsf.gov to request a tenure disapproval, mentioning review request in the subject line. If the tenure request is pending, your tenure declaration request will be disapproved to allow you access to your reviews. After you read and print your reviews, you must declare tenure again, before the May 1 deadline. After you declare tenure there will be no access to your reviews."