You can contact your Congressional representatives to ask that the whole thing be investigated. Congress has oversight power over the FBI.
But you cannot simply file suit because you don't like what Comey did (the letter to Congress? which was not leaked by Comey's office, I suspect?) and expect to get anywhere. Why not just file suit against Trump for winning the election?
There is the doctrine of qualified immunity which presents a problem:
http://civilrights.findlaw.com/civil-rights-overview/police-misconduct-and-civil-rights.html
The Qualified Immunity Defense
Defense attorneys representing a police officer for any of these claims will raise a defense of qualified immunity. This defense exists to prevent the fear of legal prosecution from inhibiting a police officer from enforcing the law. The defense will defeat a claim against the officer if the officer's conduct did not violate a clearly established constitutional or statutory right. In other words, the specific acts the officer prevented the individual from engaging in must be legally protected, otherwise there is no civil rights violation. In order to win a civil rights claim, an individual bringing a police misconduct claim must prove that the actions of the police exceeded reasonable bounds, infringed the victim's constitutional rights, and produced some injury or damages to the victim.
There is the even more basic problem that you cannot show you have a suffered an injury remediable at law. Campaign donations are gifts, not a quid pro quo for which you can lawfully claim to have expected a return sufficient to establish a property right. That sort of "donation" is known as a bribe, and it is quite illegal.
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