We are running Joomla 1.5.15, and Virtuemart 1.1.18, we received zip file, unzipped and tried to install component using com_targetcoupon_for_18.zip

For which we received the message.

JFolder::create: Could not create directory
Warning! Failed to move file.
Unable to find install package

FIxed it ... we had migrated the site and not properly updated the config file

I just tested in the install and it is working. In that release package, there is an unzipped 1.8 set of files and a zipped 1.8. Make sure that in the install step you are selecting the zip file.