There are two ways of specifying i2c addresses. It could be that there's a mis-match between your data sheet and your library (I don't use webbotlib, so I don't know how that's supposed to be set up.)
Does your data sheet specify a different address for reading than for writing? If so, you are using the 8-bit address. If the library wants the 7-bit address, you have to shift your address right one (divide by two.)
Does the data sheet just specify one address, and keep the read/write bit separate? Then you are using the 7-bit address. If the library wants the 8-bit address, you have to shift your address left one (multiply by two.)