are you by chance using multisite? multisite does not have the post via e-mail feature enabled. post via email is located on the settings > writing admin page for a regular wp install.
fyi… this features is planned for removal from core. kat hagan is working on a plugin which will replace the functionality of it. you can check out her posts on the subject.
here is the new post via e-mail plugin in beta.
about the plugin i noticed she wrote “supporting multisite is currently out of scope (though it may well work anyhow)”
The Remote Publishing was removed sometime between WP 3.2.1 and 3.5.1 … with some searching you can probably find out exactly when and why 🙂
The reason I know these two versions is because I just checked an old local MAMP install that has these two versions. 3.2.1 has it and 3.5.1 doesn’t.
If memory serves correctly, I believe that it is now “built-in” and does not need to be explicitly enabled.
