If the SD card was set up to handle apps, that means it has to be set as "Internal" storage. Otherwise it would not handle most apps. In this case, then you should be able to set that is the download location for the device and for google play store or where ever you download from. If, however, you only moved apps that are able to run on a SD card (when it is still just "External" storage) then you can't download them directly to it. Also, if you have rooted the device to accomplish this, then it may well not allow for the direct download to card either.