You're right, you need 5 rep to play around in Meta. I migrated this over for you. You should have enough rep now to post in here but you know for future reference.
Before I go any further, you can see more about how much rep you need to do things here: https://askubuntu.com/privileges it may seem silly or elitist but it's a good driving motivator to get new users active and behaving well. I realise in this case it isn't great for you.
To directly answer the question, you'd challenge a moderator or users about the reason something was closed through one of four routes:
Flag it (15 rep required). If you think it's an outright mistake that doesn't need discussion or explanation you can just flag your post and leave a short message to explain why what happened was wrong. We take flags seriously and will give you the time you need. If we still disagree with you, we'll probably write to you via email to outline what's going on.
In here. Post a thread about it including a link (the moderators make judgements on about 100 questions a day so "a unity/global menu question" probably won't ring many bells). It'll be open discussion so you may get tips from other users before a moderator sees it. Get enough high-rep users on side and you can re-open the question without a moderator at all. Mods are alerted of all new meta threads so you should get a response if you still need one.
Chat (20 rep required) is a good place to ask other users and there's usually a moderator in there depending on the time of day and how busy we are. It's less formal but you might not get a moderator.
If you don't have enough rep for any of those you have to try and find one of us by email. That's not too hard because all three moderators have personal contact details on their webpages and or profiles. Just look for the ♦s on the users page.
Now to combat the reason it was closed. Most things that are closed on the site are deemed close-worthy by at least two users. Somebody will flag it and a moderator will notice and investigate or six high-rep users will all think it needs to be closed (this way a moderator doesn't need to step in).
If that many people thought your question needed to be closed, there's a strong possibility that it's not selling itself. If they thought it was a duplicate, it needs to differentiate itself from the other threads on the site (we have tons of unity questions).
If people thought it was off-topic or time-sensitive, you might have to explain why you think it's still a valid question.
If you could see any reason why it might be considered closeable, you might want to fix that by editing it to make it clear before you ask moderators/users to examine your case. You should be able to do that while it's closed. Then ask one of us to have a look.
By the way, welcome to the site. Sorry you're having a rough start and I hope it gets better for you.
