There are situations where the inline translation does not work, because Magento can not identify the correct source string when clicking the translation icon.
You can always do a manual translation.
First you have to identify in from which module the translation comes from. By enabling template-hints you could find out that your translation is printed in app/design/frontend/base/default/template/germansetup/price_info.phtml via $this->__() and $this is of the type FireGento_GermanSetup_Block_Catalog_Product_Price the module is FireGento_GermanSetup. (there could be rare cases where this is incorrect, if the helper overwrites the module)
If you have a look at app/locale/de_DE/FireGento_GermanSetup.csv, you find the translation line:
"Incl. %s Tax, excl. <a href=""%s"">Shipping Cost</a>","Inkl. %s MwSt., zzgl. <a href=""%s"">Versandkosten</a>"
Custom translations can be added to a translate.csv in your theme (example: /app/design/frontend/foo/bar/locale/de_DE/translate.csv) or into the database table core_translate.
I suppose you translate the whole application using inline translations, so the core_translate table would be the way to go.
No in the string column, you have to prefix the string with the module name and :: resulting in the following fields to fill (locale, store_id can be copied from your existing entries):
string: FireGento_GermanSetup::Incl. %s Tax, excl. <a href=""%s"">Shipping Cost</a> translate: My own text ... Inkl. %s MwSt., zzgl. <a href=""%s"">Versandkosten</a>
A line in the translate.csv would look like this:
"FireGento_GermanSetup::Incl. %s Tax, excl. <a href=""%s"">Shipping Cost</a>";My own text ... Inkl. %s MwSt., zzgl. <a href=""%s"">Versandkosten</a>"