I have experienced this myself, it is frustrating. What I do may be called voodoo, but it works for installing Sims3Packs. First I download and extract to desktop, then run the file thru the
TS3 Recompressor. I open my downloads folder and move the file there, double click it and let it install in the game. I have found using the install button (for me) will not work. Maybe can be a solution you can try.
Well, for starters, content downloaded from third-party sites should never be installed using the Launcher. The Launcher is for EAxis content ONLY!
That is why I don't use the launcher directly. Also have a hard time installing EA content with the launcher, just doesn't work for me.