Thanks to Microsoft, there are many (like 20) DirectX 9.0c redistributables, with different policies on what DLL's are included. While one version might have _29.dll and not _28.dll, other versions might have _xx.dll and all preceding numbered DLL's.
No offense, but unless you explictily tested a specific version of MGE with a specific redistributable on a vanilla PC, there is a very good chance that it won't work. IIRC my MGE required _43.dll or something and the latest redist didn't work for me.
The exact DLL you mean is D3DX, and yes, there are many versions (it's into the mid 40s now, or so).
However, the DirectX redistributable, particularly the full offline version, comes with most or all of the files required (both the current D3DX modules and previous versions). Windows and Windows Update don't always get the full pack, just the basic DirectX files, but the redists come with quite a few more. If you want to check exactly which are included, download and extract (but don't install yet) the package, and you'll see the included files.
In addition, MGE is traditionally built against the latest redists, which have been June 2010 for some time now. When running into DirectX issues, the first step and usually only one needed is to download the full redistributable package.
Edit: Oh, and there's actually one "policy" on what goes in the redist packs, it's just been updated over time (adding new files) and there's something of a "light" version that comes with Windows by default (missing a ton of stuff). The slim version and the fact 7 doesn't come with the DX9 files is a huge cause of trouble.