Do you want to edit a directory in /system? Or do you want to create a file in /system?
Then you need to mount /system rewritable. That is possible with this app!
With this app you can mount /system rewritable or just readonly.
The only things you will need are Android 2.3+ and root rights.
Do you want to remove the ads or donate, then you can buy the pro version here: http://bit.ly/MountSystemRORWPro