Edit the page in WYSIWYG mode, place the caret inside the table and select “Bordered Table” from the “Styles” drop down that you have on the CKEditor tool bar.
The editor that appears in your screen shot is the old GWT-based WYSIWYG editor that has been replaced by CKEditor in the latest versions of XWiki. So you’re not using CKEditor witch means my comments from this thread don’t apply. You can still use CKEditor (it supports XWiki 6.2.5+) but you need to install it with the Extension Manager from the Wiki Administration. See http://extensions.xwiki.org/xwiki/bin/view/Extension/CKEditor+Integration . Or, you can simply follow the FAQ https://www.xwiki.org/xwiki/bin/view/FAQ/AddTableBorders .