How do I get iTunes to look for music on my computer

Question

Here's the problem: So on my old HP laptop I had around 5500 song files saved/stored in various locations all compiled for easy access through iTunes, as in I could access all of them there; standard, no problems, what everyone expects.
On it's last limb I completely backed the computer's C drive contents to an external harddrive. What I didn't expect was that there would be no organizational structure of the backup: merely 3 folders each containing a series of ~50-300 subfolders of backed up content (zipped, at that time, mind you). Around 88 GB in all, which is interesting because it was 140GB in size on the hp before being backed up.
Towards the process of unzipping all of them, I unzipped only the very first sub folder which contained around 50 song files (this isn't to say each folder just has songs; there's tons of other random files ranging from documents to programs to random coding) before opening iTunes for the first time.
This part is critical to understanding my issue. When I opened iTunes for the very first time and it was initializing its components, doing a startup etc., I had only unzipped that one folder; none of the hundreds of others had yet been unloaded onto my mac's harddrive and viewable as downloads.
During the guided installation process iTunes prompted me at one point and asked if it wanted to "search for music already on this computer" or something like that. I said yes and it seamlessly uploaded the 50 or so songs.
The problem is I wish I had waited until I unziped everything because I don't know how to access that feature again, in that I do not want to manually dig through each download folder and upload songs individually when I know there's a feature that can look through my computer's hard drive for me and upload mp3 and similar files in a fraction on an instant. For example, when I downloaded and opened Spotify, it did this on it's own (although it included a bunch of duplicate files so there's around 7700). Some may question why I don't just use Spotify for my library then and I'd respond by saying I don't like it, I'm not used to it, and iTunes is more compatible/easier to customize, create playlists, sync with iPhone, iPod, etc.
I've even tried completely uninstalling and reinstalling iTunes (using this method: http://www.iclarified.com/entry/index.php?enid=4045) to no avail; once I opened it again for the first time after reinstallation it already has included only those ~50 songs and doesn't prompt that "search for music on this computer" feature or whatever. I actually think that feature accompanied my mac as a startup, when I started the computer for the first time from factory settings.
I'm hesitant to completely wipe my computer because one, I've already been using it for a couple weeks and don't want to do another back up (I'm not very mac savvy if you can't already tell) and two I don't even know if that will work.
I want all my songs from my computer all accessible to one area: iTunes. Please help!

Answer 1

In itunes preferences, Advanced make sure the two following options are checked
Keep Itunes Media Folder organised
Copy files to itunes Media folder when adding to library
The box above that should have your iTunes Media folder path
/Users/username/Music/iTunes/itunes Media.
If you still have the original 50 songs on your harddrive delete them form itunes. Then drag your external Hard Drive icon into itunes  and it should identify and copy all your music files to your new machines.
Check that everything is there then you can do a freah backup to the external drive with everything in its correct folder