The files you sent work perfectly. The .xml file is recognized as a .net symbol file with comments. The .dll is recognized as .net CLI DLL.
The .dll file has two namespaces (UnityEngine, UnityEngineInternal). The .xml file has two namespaces (System, UnityEngine). Tons of symbols under these.
Go to Tools>Tag Files to create a tag file with these symbols.
Press the "Add Tag File..." button to create a new tag file. Choose the language C# to add the tag file for C#. Specify an name like unity.vtg.
Use the "Add Files" button to add the files to the tag file.
In order to view your files, you will need to change the File Types in the open dialog. Just type "*.dll" or *.xml" to view one of your file types.
Once you added your files to the tag file, you can view the symbols in the Symbols tool window by expanded the tag file you created.
Just ask if you get confused creating the tag file.