Registering a DLL is totally unrelated to DLL overrides.
In the screenshot above registering worked (that's what the "exitcode 0" says); That means (to simplify, I don't know the details anyway) that the DLL itself published in the registry informations about what it's capable of, so that other programs can call it from that moment on when they need specific services. For example, a video codec could publish what format(s) it supports, so that it can be called to decode (or encode) files in additionnal video formats.
Overrides is a different mechanism of Wine, not Windows; It allows to specify, when both a Wine and a "native" implementation of a DLL exist, what takes precedence. By default it's Wine version that's used, but with overrides you can decide that the native DLL will be used instead, or that you want one to be used then the other as a fallback if the feature looked for is missing.
Using native DLLs usually require both override and registration, but they're separate mechanisms.
So, in the overrides don't expect registration to create an entry; Instead just go ahead and type "dsdmo" and add an override for it.