you've probably selected either a wrong patch version, or the wrong language version of that patch..
there're different versions for each language + region,
so german, french english etc use a different updater of the same patch-version,
and further australian, english(uk-int) and english-(USA) have different versions as well..
if your current patch version is below 1.8 or equal 1.7,
you'll need the 'full' update ( +130MB) for patch 1.8.x6
..if you already have the 'plus' version or installed that update, you only need the 'small' patch (~10MB) for 1.8.x6
for Underworld, iirc it's less confusing for owners of any english version,
as there's only one 2.28 update.. (eng_enu)
also make sure you've made a proper install of the original sacred, with all registry entries intact and the sacred folder in the corresponding root folder you've selected on install,
before installing the addon, or any other patches
I'm using Sacred Gold which has both campaigns in it.
So the 2.28 patch is the appropriate one.
I had the game installed under the directory program files/games/sacred
but it the patch couldn't find it.. so saved my game files, reinstalled under the directory the game wants.
applied the patch again but 2.28 cannot find my game.
Of course it helps if I looked at the version of the game before I do this.
The gold is already 2.28.
*laughs at herself*