I have applied for a visa to attend a conference to represent a non-profit organization at the United Nations and but the visa refused?
Some countries have strict visa rules. You need to research the requirements.
This means USCIS doesn't feel you have sufficient ties to your home country. This is the standard for obtaining a visa for any nation.
The biggest concern of US immigration these days is that people not come in on a legal visa and stay. Any evidence you have that you will return to your country will help- being on leave from a job for a short period of time, for example, or owning property, having family ties, etc.
Sucks to be you. Now, try to ask a question.