In general, at this time, there is no federal expungement or sealing provision, such as the various State's have. You can seek a presidential pardon (although the process is not short or easy) but that will not, by my experience here in NJ on behalf of our clients, give you the relief I believe you are looking for, which is a clear record (or the ability to say you have not been convicted). As a starting point I suggest you review my legal guide on Presidential Pardons at http://www.avvo.com/legal-guides/ugc/no-federal-expungement-what-about-a-presidential-pardon and review the information at http://www.usdoj.gov/pardon/pardon_instructions.htm
A bill to permit federal expungement in certain circumstances was introduced in 2005, but seems to have gone nowhere. Check it out at http://www.fedcure.org/documents/HR662.shtml I suggest you and others similarly situated need to get behind these types of bills or they are not likely to get done, I suspect because they are typically seen as being 'soft' on federal crime legislation. I recommend to our clients that they and their familiy/friends need to get vocal about this with their congressmen to get the issue back into the arena of discussion. I also suggest that our clients start establishing a strong mitigating basis to show potential employers that they are trustworthy (for example by developing a strong community service background).
In addition, iinteresting commentary pertaining to expungement can be located at http://answers.google.com/answers/threadview/id/478249.html
I suggest that you get a copy of your entire file, especially the disposition sheet indicating the specific statutory citation that your conviction was entered under, and directly consult with an experienced criminal defense lawyer admitted to practice law in CA (there are a number of excellent ones who list here on Avvo.com) to see if there is any other alternatives open to you, and to confirm the fact that you are not able to expunge this conviction. Good luck.
Since I do not practice law in your State, this answer is provided solely for informational purposes only, for you to use as a starting point when speaking directly with a lawyer in your State. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising. I urge you to immediately contact an experienced criminal defense lawyer admitted to practice law in your State before you make any decisions about this case.