CKeditor as part of the wb-package or as single download (maybe) directly from ckeditor.com?
is only the link button missing or the complete function - the link-dialog?

the icon is defined in modules/ckeditor/ckeditor/plugins/wblink/plugins.js
default icon for un-used links is modules/ckeditor/ckeditor/plugins/wblink/icons/wblink.png ->

if the editor founds a working link, the icon change to a light-blue-icon, filename: _wblink.png
please check at first, that you have this file in the folder modules/ckeditor/ckeditor/plugins/wblink/iconsif the popup-windows doesnt starts after a click on the (in your screenshot empty) link-button, maybe the plugin is deactivated.
if you use the ckeditor from the wb-2.10.0-package, the plugin there is activated in
- modules/ckeditor/include.php as part of this array -> $ckeditor->config['extraPlugins'] (~Ln 181)
- check here also the next array -> $ckeditor->config['removePlugins']. these are inactive plugins, so be sure, that wblink is not a part of this array
- modules/ckeditor/wb_config/wb_ckconfig.js
- check, the the name "WbLink" is Part of the used toolbar. default toolbar is "
WB_Default", but its possible, to change this default toolbar with admin tools like
wysiwyg_admin etc
- check also the config array's
config.extraPlugins and
config.removePlugins - wblink must be a part of extraPlugins and not of removePlugins
- custom wb_ckconfig.js in the used frontend template(s)
- same settings like the default wb_ckconfig.js, but for separate settings in every template
(in theory its possible, to give different pages or section types a different template with the same design, but with other settings for the editor. For example: some special pages are only for the admin and he gets a extended toolbar, but the news-Page has multiple autors and needs only a small toolbar - thats the idea behind the custom config files in template folder
last point: the whole editor works with javascript and sometimes, its difficult to clear the cache. use a tool like ccleaner to clear the cache
and!!
go to a page without wysiwyg editor (maybe media or settings), clear the cache and go back to the page with the editor
P.S.: use only editors, designed for WB