Which version of ACID do you have and when and where did you buy it?
I don't own ACID so can't advise on the location of the loops etc., but are you certain you are entering the serial number exactly as it was given to you in the confirmation of purchase email you would have received? And the email address must be exactly the same as the one you used when purchasing the software.
One other thought: all recent Magix software products have a serial number starting with P3. if yours starts with any other number or letter you almost certainly ahve a voucher code, not a serial key. To redeem a voucher code please follow the instructions on this page.
If none of the above helps, please post a screenshot of any error message you receive, or at least the full text of the message.
If you are certain you are entering both the serial key (beginning P3) and your email address exactly as shown in that confirmation of purchase email your only option is to email infoservice@magix.net, explain the problem and ask for their assistance in resolving the issue.
Just taking a step backwards, since you say the serial number is not accepted and therefore the software is not fully registered and activated, I think the additional downloads of loops etc. may not be available since they become accessible to download only when registration and activation is completed.