The tab should only have two positions, read only and free. You can try to format the card, look at page 25 here
http/gdlp01.c-wss.com/gds/7/0900011247/01/PSSD400IXUS55CUG-EN.pdf
The tab may be broken, try the card in another camera (newer camera is best due to issue I outlined bellow), or in a PC card reader.
Another possible issue you may be having is that the camera is from around 2005. There were no cards close to 8 gig back then, the camera may not be able to read the card at all. It came with a 64MB card when new. Try to track down an older card under a gig and try that. Or maybe one of these 2 gig cards
http/www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007962%20600006188&IsNodeId=1&name=Secure%20Digital%20%28SD%29 although those may not work either.
The largest card offered for that camera from Canon is 512MB, see page 178 of the manual I linked.