Welcome to the forum. You probably should have started a new thread instead of jumping on a thread last used 7 years ago, but maybe a Mod will come along and move it for you.
Welcome to the Mac world. In macOS, there is a concept of "ownership" of files built into the security of the system. Ownership means that the owner of a file or folder is the only one authorized to open/edit/save/delete that file. What you did that created the challenge for you is that you created a user first, then you used Migration Assistant. Because the "owner" of the migrated files was not you, the system put the migrated files in the shared user so that at least you can see them, even if you cannot do anything else with them. What you should have done is to use Migration Assistant when it was first offered in the startup routine, before you created an account. That way MA would have put the files in an account you could get to and do whatever you want to because you would be the owner. MacOS is based on a linux kernel, which is a multi-user system from the design, which means the security is seriously better than Windows.
The easiest fix, if there aren't too many of them, is to go to each of these greyed out files and right click (or Ctrl+click) on the file and then pick "Get Info" from the menu that pops up. At the bottom of the information window about that file is an icon that looks like a lock. Click on the lock and enter your admin password when prompted. Now, just above the lock is a block with "Sharing & Permissions" and a list of users. Staff, everyone and any user that has access in any way. To add your account, click the "+" and select your account name to add you to the permissions list. Then adjust your permission with the up and down carets to be "Read & Write." Now you should be able to move the file from the shared account to your own.
If you select a folder and follow that process, once you have added the user you want and selected "Read & Write" you can click on the little circle with a down caret beside the + and - buttons below the box and see an option to "Apply to enclosed items" that will fix everything in the folder at one time, saving some effort. That may make it easier for you if MA put most of the files into the same folder structure.
Now, if there are too many files for you to want to do this one at a time, there is a way to recover, but it's a bit painful if you've done much with the Mac. You can reinstall the Operating system completely, erasing everything on the drive so that on boot it goes through the initialization process again, allowing you to take the Migration Assistant offering when first made, before you create an account. At the end of the initialization and migration your files should be in an account that you can get to and not have to move them. As I said, painful if you have any new files because they will be erased in that process, so you'll need to back them up somewhere while you do the reinstallation. Frankly, that would be my last resort choice.
One question. Did you get a Mac Pro (a desktop machine) or a MacBook Pro (the portable machine)? It's important to use the right names and terms so we can get you any future help you might need better.