I am not sure if this will help you, but I was able to solve the problem you discribed.
I looked at the backup file that was created when I did the backup, which was named as the following:
In my case, I had backed up my c and d drive to a backup directory on my d drive as backup (thus d:\backup). The above nbi file was in my d:\backup directory and inside a sub directory was my c and d sub directory. Thus my d:\backup directory looked like this:
in d:\backup was 20052710_193622_tom.nbi
as a sub directory in d:\backup was the following:
and within the 20052710_193622_tom sub directory was my c and d directory will all of the compressed files and sub directories that were created when the backup was first performed.
Thus I had to re-create a directory called backup on the d dirve and all of the sub directories like you see above.
Once I re-created the directory structure that was used to do the original backup, I then attempted a small restore to another location (in my case, I did it to c:\restore). And presto, the restore worked. Before that, I just had the d:\backup directory, along with the nbi file and a d sub directory and the compressed backup contents stored in the d sub directory.
Thus what I am saying to get around my error (The file ‘file name’ is not available in hte backup folder. Please select the parent of your backup folder) was to create a sub directory in my d:\backup directory that matched the name of the nbi file without the file extension. I was kind of lucky in that I could remeber where I did the backup to (in my case d:\backup).
If you can remember where you had your backup files and the nbi related sub directory, you can get around this error. I hope this helps, as I had been pounding my head against the door trying to figure this one out.