It's been a little while since I used Scintilla, but my understanding is that SciLexer.dll is an entirely native DLL, which is why you can't add a reference to it from Visual Studio.
Instead, you should 'arrange' for SciLexer.dll to appear in the right directory at runtime so that it can be loaded by ScintillaNET.dll. The safest way to do this would be to add SciLexer.dll to your Visual Studio project as a plain file (not as a reference), then change the properties on the file to have it copied to the output directory when your project is built.
Given that you're on 64-bit, I expect that you'll need to build your app specifically as 32-bit and not AnyCPU. As I say, it's been a while since I did this, but when I did, Scintilla only had 32-bit binaries.
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