There are some known issues with the version of CKeditor that shipped in mojoPortal 2.4.0.0 and IE 11. They released an update to CKeditor with some fixes for IE 11 on the same day we shipped mojoportal 2.4.0.0 so it did not get into our release but we have updated in our source code repository.
We will be shipping a new release of mojoPortal very soon that has an updated version of the editor. In the meantime I suggest try a different web browser or use TinyMCE