RBOSSE had the right answer for me, I think what happened was the initial download did not complete or corrupted, and there is no clean way to tell Unity to re-download it (the only visible option is to Import). I manually removed the bad download and re-ran it from the Asset Store and it worked.

I'm pretty sure this is a memory issue - the blacksmith packages etc are massive and windows explodes if it gets low on memory - since unity is loading and unzipping the entire package directly in memory - the package just makes windows asplode.

all of your systray icons, hidden apps etc all explode into a million console windows and you basically have to reboot to fix it.

great fun. short version: Unity should unzip to disk instead of trying to open the entire package into memory.

The issue is the store shouldn't be letting you import when you haven't got a download. @RBOSSE is correct, its just that the store shouldn't even give you the option to import if its not downloaded.

@RBOSSE: When it happens to me the package downloads fine and is in the roaming folder. It crashes right after the download completes. In the asset store via Unity it shows as downloaded and I can try to import without re-downloading it.

Verify the package file exists. I just had this happen to me and noticed it wasn't there however in the Asset Store via Unity, it shows it was downloaded.

Location of packages for Unity 5.X:
C:\Users\<USERNAME>\AppData\Roaming\Unity\Asset Store-5.x\

Re-download and it'll import fine.