The main problem is that I can't write posts to threads and it doesn't
show me some toolbox for radeditor. Can you suggest me something in
If this happens on the front end, check whether the user you have authenticated with has needed permissions to write in the forum. You can also check the settings you have for Moderate posts and Deny anonymous access. The second one will require login.
Also can you explain how I can change skin for RadEditor for forum?
It depends on which template you are working with. You can use one of the default templates specifying Skin property and setting EnableEmbeddedSkins="True"
the Telerik team