The temporary decal mirror doesn't have all of the supplementary files for decal installation. Namely, DirectX setup and .NET 2.0.
You can get the DirectX setup from here:
Web Setup (recommended) or
Full Download. Even if you have DX9 installed, you may not have the required DX9 update for Decal.
Use
Windows Update to get the .NET framework 2.0 if you don't already have it installed. If it's not listed on Windows Update, then you already have it.
After you've installed those two items, you can get the Decal installer from
http://decal.dreagis.com/Decal.msi (official mirror). I believe that version doesn't have the pre-req checks, and won't fail when it can't download the DX installer.
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[LotRO] Digero (Guardian), Digrim (Burglar), Dignite (LM), Azrea (Hunter) - Landroval
[AC] Digero, Lyera, Draxxe - Leafcull (Retired)
[CoH] Devil's Zealot, Scinta, Izzard - Guardian (Retired)
Digero's AC Decal Plugins:
http://decal.acasylum.com