You have to learn how to use Flash, and it takes time to do that. If this gallery is key to keeping this client then you will probably do best to hire an experienced professional to do this for you. Chances are they will have a gallery that can be modified to fit your specs. If you do not want to afford hiring someone, then you will have to chance finding a tutorial online that will walk you thru the process. If you don't already own Flash software, you may want to reconsider hiring someone because it will likely be less expensive than purchasing Flash.
Please don't crosspost in these forums.