Try inserting that SDcard into a card reader on your computer/laptop, etc. and format it to the FAT32 file system. Most navigation systems won't recognize cards if you don't format them to the old FAT32 standard.
Insert the card and then right click it in My Computer. Select Format, and choose the file system to Fat32. Additionally, if that cards needs its own file system, then this procedure won't be available. Contact the seller and he should provide additional information related to the space problem.