Export the table as a dBase file then you can incorporate it into a layer on your map.
I am having trouble with this too. I use an mdb file because it allows null values. I tried exporting to dBASE as suggested but it converted all my null values to zeroes.
If I go the .csv route, will a ",," be recognized the the data is missing, skip to the next field?
P.S. I just tried this. If I create a column with a couple of cells in which I never entered data, it loads into arc gis online as missing. However, a column with such missing data can't be used to create a size-gradation symbol map. If I go back to the csv and delete a value from a cell, then reload the csv into arc gis online, that cells loads in as a zero, not a missing value.
Unless anyone has a better idea I think I will convert the mdb to a shp file, then go in and edit the attribute table, changing what should be a missing value to a large number. Then I can create the symbol map and change the symbol for that size class to something different, and chage the label for that size class to "no data"
