Adding XY has barely changed so it should be fine. Check your table for illegal characters, spaces in the headers, empty coordinate records. Add the table to ArcMap using Add Data, right click on the table in the TOC, select Display XY Data and populate the dropdowns. Your data is in decimal degrees, but do you know the datum? You should also define this in here if you know. Hit OK and save your layer if you want.
If you can't determine the datum the data is pretty hard to use with any confidence. It probably depends on the required accuracy, and the accuracy of the input data. Quite often I get decimal degree data with 2 or 3 decimals. I am sure people giving me this information rarely realize that the difference between 51.72 and 51.73 is over a kilometre, the difference between 51.720 and 51.721 is over a hundred metres....etc.
If your source data has 2 or 3 decimals and falls roughly where it should when overlain on ERSI sample data (or any geographic data with WGS/NAD datum) then you are probably ok to define the datum as NAD83 or WGS84, but really guessing is not my preferred route;)
