Without having more details, the best I can suggest is that you go back to the logic level converter and do some more troubleshooting.
A few questions that will help us help you:
- What IC are you using?
- Does the conversion from 5V to 3.3V work if you just hook up a 5V signal and the appropriate power supply?
- What about from 3.3V to 5V?
- How fast is the I2C bus running, and can your chip handle it?
You're going to need an oscilloscope if you don't have one, and maybe a function generator. If you don't have access to these, seek out some mentors from a local university or technology company that might.
This Sparkfun board might work, but unless you know what your current problem is, don't expect to plug it in and have everything "just work".