I've noticed that in some very rare cases, when using Migration Assistant, it may move your stuff to a different location.
Check your hard drive (by clicking the Finder's icon on the left side of the Dock at the bottom of the screen, and in the window that opens, there should be an option for "Macintosh HD" on in the sidebar on the left side of the window) and see if there's any folders there. They may say something like "Users (from old Mac)" or "Applications (from old Mac)"... something along those lines.
If there are folders that have those names, your files are inside of them. More specifically, your personal stuff (documents, pictures, etc.) would be in the folder called "Users (from old Mac)". You would need to take the files out of the "old Mac" folder and put them into their respective, new folders.
Meaning, for example, that you would need to take your "Documents" folder from your old Mac, and drag all the files out of it and into the new "Documents" folder on your new Mac. Does that make sense?
The same would go for your Pictures (just take the contents of the "old Mac" folder and drop them into your new Pictures folder), and your Music, and anything else.