define frequancy
You need to create an input data file (.pts). You can easily create it by putting two rows in Excell: 1 row is the Frequency and the second row is your Data. Then, you copy those two columns into Notepad. You may have to change decimal "," by ".". Finally you save your text file as .pts.
Then you can enter this data file in HFSS by importing from a file when adding a dataset (Menu Project-->Datasets...-->Add...->Import from file... You will see your data plotted in the graph beside. Once you have your dataset, you can use it as a frequency dependent property, for instance, frequency dependent electric permittivity. Check also HFSS help.