Troubleshooting when you can't find a class
While working on the project, open the "Object Browser" from the "View" menu.  Then in the textbox labeled "Search" type in "Islem" and see what comes up.
If the class doesn't come up at all, you're in the wrong project, or you didn't add the class like you think you did, or it's a different project and you need to add a reference.
If it does come up, take careful note of the class' complete namespace.
My guess
My guess is that you have a mismatch on the namespace.  By creating the folder you created a sub-namespace by default, so you probably need to add something like this to the top of your code:
using Deneme.App_Code
or, if you don't want to add that for some reason, you can instantiate an instance of the class using its full name:
var o = new Deneme.App_Code.Islem();
Another guess
Make sure Islem.cs is set to compile.  See this answer for instructions.