Oh I see. It does sound like because you redeemed a different promotion for your product they're not letting you redeem this one. I assume when you contacted MSI you did it via their "Ask a question" page and chose the "Promotion: Game Bundle and Hardware bundle" option followed by "How to get the promotion"? If you did this, was their an option to select your product?
Something like this:

Hey - thanks for your reply.
Yeah, I contacted them via the "Ask a question" page selecting that category and even attaching my product receipt with dates and all, so they could see I bought it on the 3rd of April. I also mentioned that I redeemed the software pack promotion because this new one wasn't available on the 3rd when I got the computer.
I'm not going to copy/paste the answers I got from support, but they were... not very adequate. In both answers I got they just simply directed me to the promotions page and told me that only products bought from 1st of April to 31st of May were elegible, basically ignoring 90% of my questions and attachments.
I also think that since I redeemed the software pack then I can't opt for this new promotion, which is a pity because I don't care for the software pack and I could have received a better reward had I waited a couple of days after purchasing... but I guess I can't complain about freebies so I'll just have to suck it up.
I wish the support team was able to answer that question directly, though.
Thanks again for your time.