crm 2011 - custom query field

Hi

i need to do a simple thing but cant figure out.

i have created addon for Crm called "donations" It holds the data about who donated how much. it has fields like (donationid,donation time, amount, donatorid<---should be lookup from accounts)

when importing donations it should query Accounts for donator_id and in case of mach should make a connection between account and donation.

i can by default only lookup for account name. How can i lookup for donator_id field that i have created on accounts page?

thanks for help

Kuuno