You are basically missing a "T" :) But there is also an extra "d" there, it is "TInternalNameRange" instead of "InternalNamedRange". The extra d in name"d"Range is why you couldn't find it with GExperts.
I've already fixed the docs (which won't make it to 5.6.5 because it is already uploaded and waiting for release in the website, I hope tomorrow).
Sorry about that, normally the docs might miss the leading "T" because much of it comes from FlexCel.NET, where those "T" aren't there. (And there is a lot of docs to review, some parts might slip). In this case, you could have find out with APIMate (which gives the correct "TInternalNameRange"), or by pressing Ctrl-Space in the arguments of GetInternalName, it suggests TInternalNamedRange.
Again sorry about the typo, but I'm really glad to know it is being useful :)