In order to get a visa to Nigeria, someone in that country has to represent you and send you a letter of invitation that you will need to take with you to the embassy along with a travel itinerary for your dates of travel. For example, if you want to go to this conference or if you want to go around the same time as this conference, but not actually go to the conference, get the conference to send you the letter of invite with your intent to attend.
I would contact the Marketing or point of contact person in charge of this Conference, let them know that you want to attend, but would need a letter of invite. In your request, I would e-mail them a "dummy" travel itinerary (flight) until you are able to secure your flight. I've done this several times, but usually, my letter of invites were from political parties.
For businesses, it works the same way. The rep from that company will have to send you a letter of invite on their letterhead with their contact information and the letter must stay why you are there (i.e., business meeting with client, etc.). They don't actually check with the person sending you the letter, but it just ensures that you won't go to their country, and let's say you wild out partying and have a coke induced fit and mess up one of their hotel rooms or get arrested. The person repping you in the letter is responsible.
I hope this make sense. Let me know if you have questions.