Unhide Partitions via Terminal
Often "hiding a partition" is simply setting them to not mount at boot. Have you looked in /etc/fstab for a noauto line that prevents one of the Volume UUID from mounting in the default manner?

You should be able to read all the partition, volume and filesystem information in terminal or Disk Utility (both of which are in the Utility menu of the install app)

diskutil list, diskutil info disk0, and diskutil info disk0s2 should get you started.  diskutil will list all the commands if you just run it without any commands listed.

Of course, if the utility is using non-standard drivers or encrypting/obfuscating things you will have a harder time getting into the drive without more detailed information on how that specific utility did it's work. If diskutil can't see the partition then it's not actually hidden but gone. Tools like that run the risk of you using the space that is marked as free unless they do a good job of modifying the size of the device and hacking the partition table data.

In response to your edit - if the partition tables are changed to be non-standard you can undo the damage by being very good with writing hex values using dd  - if you get the it right, you can reverse the changes that were made to alter the file system definition data. Using gparted or another tool that is built for the task would sure be safer and probably easier.