That is because the SD card is not considered the 'space' that your phone is telling you is low. I am guessing at your question, since you kind of put the title where the question should go, and the question in the title, so it got cut off.
Alas, in that version of Android, you can't make the SD card "Internal" storage, which is what is needed to run apps from it. You can, however, move other files (just not apps) over to the card to free up space.
If you want to just move files (photos, videos, music, etc.), not apps, to the SD card you can use the file manager app that is on the device, or if there isn't one you can download and install one to access and move the files. Sadly, they usually only let you move one or two at a time.
To move them all at once, or in large amounts, you can connect the phone to a computer via USB cord, then move the files over to an empty folder on the computer. Once there, and still connected via USB, you can then move or copy the files over to the SD card. Personally I would suggest leaving a copy on the computer as a back up.
Once that is done you should then go into the apps that you access the files through (like gallery, etc.) and change the apps individual settings to look for the files on the SD card. You may also want to change the settings in apps that you use to create new files (like the camera) to save new ones to the card rather than the phone.