This has been answered more times than any other question on the forum i think
Rather than say, use your search to find the answer - which it is always best to do before posting - simply look for a line in skin1/customer/main/product.tpl and replace:
where it says producttitle in
{include file="dialog.tpl" title=$product.producttitle content=$smarty.capture.dialog extra="width=100%"}
with product so it reads
{include file="dialog.tpl" title=$product.product content=$smarty.capture.dialog extra="width=100%"}