Displaying code snippets

I am confused.

I am trying to show code in my blogs. So I installed your plug in. I start a new post. I click “Screen Options” and tick the “Custom fields”.

I add a new custom field “CODEXYZ” and put some code/HTML in the value.

I use %CODEXYZ% in my post and click preview. Just shows some of the text. 🙁 I find it all to hard still. All I want is to embed code. What am I doing wrong?