I'm Using .Net Reflector with Reflexil to edit or view .net Assembly codes.
I see this picture in reflexil.net:
It Shows the "Compile" window. How can I see this window and edit codes?
Decompile a function with .NET Reflector, and in the Instructions tab of the Reflexil window, right click on an instruction and choose Replace all with code...:
This blog that I wrote a a few years ago describes all the steps to modify a .net binary with Reflector and Reflexil. Not sure which steps you're missing but I recommended to read&compare.
However, Today I would recommend Telerik's JustDecompile though as it has built in de-obfuscation (de4dot) and has an option to replace a section with code which is much, much, easier than writing replacement code in IL.
/EDIT: Here's a blog I wrote about using JustDecompile and Reflector to modify a .net binary.
There is a nice tool to play with .NET assemblies (https://www.digitalbodyguard.com/graywolf.html)
GrayWolf – Is a reverse engineering tool (focused on .NET Framework Applications)
Currently - is in a late beta stage It is similar to IL-Spy but this has a focue on edit/attack. GrayWolf was a tool I created to carry out research. The focus is on editing applications to make change quickly.
What can it do: DE-ObfuScatE,Edit IL(Live), Add payloads, Edit attributes(public/privet), Copy strong names signing on EXE/DLL