I know you say you've already done the trouble shooting steps I'm going to suggest, but just bear with me for a few minutes, if you don't mind.
1. In iTunes Pull down Store > Turn off iTunes Match.
2. Pull down Store > Deauthorize this computer.
3. Log out of the iTunes Store.
4. Quit iTunes and just for good measure restart your computer.
Now we're going to reverse these steps.
1. Start the computer then launch iTunes.
2. Log into the iTunes Store.
3. Pull down Store > Authorize this computer.
4. Pull down Store > Turn on iTunes Match.
You should see a new entry in the left-had column, under Store, called iTunes Match. If you click this is will show you the progress of the process. You can also enable the iTunes Status column in the iTunes Browser to show you what the status if for all your music. Pull down View > View Options, select "icloud status" and click OK.
There's also several tips for using Smart Playlists to help you manage the scanning process in this forum. Post back and let us know.
Hey, thanks heaps for looking into this for me, I sincerely appreciate it.
Unfortunately, I've just tried the full deauth as you've suggested and when I got to step 4 in the reverse order the same thing happens- clicking on Store > Turn on iTunes Match doesn't seem to do anything. I checked Activity Monitor too, in case initializing the matching process was giving it a hernia or something, though everything seems normal (my library is only 2k odd songs anyway). Not sure if it matters but it's interesting that the in my Music pane, the iCloud Download column shows the dotted outline of a cloud next to each track, which I cannot click, and the iCloud Status column shows 'Waiting' for every track in my library. I've tried nuking the entire library and re-adding single songs / albums too.
I believe there may be another account authorized on my Mac, an ex, and I don't have her password. Whether or not this might be interfering with activating iTunes Match, I'm not sure- is there a way to wipe all current authorizations without having the respective account details/passwords? At this point I'm considering booking a Genius Bar appointment, though I've a feeling they might just end up wiping my Mac which is also an option I suppose.
Again, thanks for the help.
One more thing- to clear up what I mean by 'nothing happens', the iTunes Match entry I garner I'm supposed to see doesn't appear in the column on the left-hand side of iTunes. No processing/action seems to happen, I've tried leaving iTunes open for ~20 minutes after clicking it and nothing transpires. It's as if the menu option to turn on match is broken.
A-ha! Worked flawlessly on a PC, and also when I created a new user account on the Mac (using the same apple ID of course). It must be something corrupt in the installation of iTunes, which is odd because I tried removing it completely and starting fresh already. I used AppCleaner last time, for lack of a better, known alternative. Do you have any advice/guide on doing a complete reinstall? If not I might just follow this old guide I found:
Before doing all that to iTunes, try this.
1. Restart and hold down CMD+s. This will boot you into single user mode.
2. When you see a command prompt type fsck -f and hit enter.
Let this process run until finished, you'll know when it is done. If it reports the file system was modified run it again, just for good measure. When it's done type restart and enter to get back into OS X. This should help; post back and let us know.
Okay, used AppCleaner to remove iTunes, followed the iclarified guide posted above to cover anything that it had missed, did a manual scour of anything else iTunes related in Spotlight, trashed everything. The only thing I left were my mobile device backups in the MobileSync folder.
Clean install of iTunes 10.5 and iTunes Match appears immediately, everything working perfectly.
Thanks heaps for all the help Michael!