Try the free decompiler, dotPeek, from JetBrains: [
^]. Or, try Microsoft's ILDASM that comes with .NET [
^].
Long ago, Lutz Roeder's Reflector [
^] (free) was the tool of choice for dis-assembly, but then Red Gate Software acquired it, and it is now a commercial product with two versions with different features available [
^].
ILSpy, open-source, free, is a new "incarnation" of Reflector [
^].
But, best of all, would be to get some information from whoever developed the dll: hacking is not its own reward !