Some things to watch for:
1. Longitude values are negative in the western hemisphere. Latitude values are negative in the southern hemisphere.
2. Longitude corresponds with x; latitude with y.
3. The values have to be doubles/decimals, not various display formats like DD MM SS.sss, DDMM.mmmm, DD.MMSSssss, etc.
What coordinates do you have, and what do they show up as in ArcMap?