Effective April 2015 the Ethiopian Government requires all business visa applications to be preapproved before a visa can be issued.

Pre-Approval Process
You must obtain pre-approval in order to travel to Ethiopia on business. Your host or sponsor must contact the Ethiopian Department of Immigration or the Consular Department of the Ministry of Foreign Affairs on your behalf. They must request that this pre-approval be sent to the Ethiopian Embassy in Washington DC.

Copy of Visa Pre-Approval
You must provide a copy of your visa pre-approval from the Department of Immigration in Ethiopia. Your visa cannot be processed without an original approval sent directly to the consulate and a copy of the approval
included in with your visa application. The Pre-Approval will identify the type and duration of visa the Embassy will issue to you.