You can set the Kartris defaults for the meta-tags using the following language strings:

ContentText_DefaultMetaDescription

ContentText_DefaultMetaKeywords

In places where no other data is available, Kartris will fall back on the values here. But in most cases, it will find more page-specific data to use if possible. As mentioned earlier, the meta-keywords tag is not read by Google and most major search engines, so is probably best left blank so as not to increase page sizes unnecessarily.