The path to the suspect file is reported in the actual alert, but as has been said it is hidden from the usual windows explorer interface and just one reason why it is/was considered suspicious.
As an avast user I can only offer what is my best guess:
Most of the rectification action will be from the CommunityIQ feature in avast, reporting information about detections/suspicions being passed back to avast. Over a short time this should show a pattern, which should be investigated.
It is possible to report it by email to virus (at) avast (dot) com as a possible False Positive in the subject and details in the email body based on the file name, its location and what detected it (anti-rootkit), e.g. all the information from the alert.