Solved! Go to Solution.
I think you could do this:
1) load your Excel file in ArcMap
2) right-click on the table-layer in the 'Table of content' (TOC)
3) chose the option ''Summarize'', then summarize base on the ZIP code field. This will return a new dbf file with frequencies of each ZIP code. (maybe you will need to transform the Excel file in DBF in order to summarize..I don't remember..
4) you could then join (by attributes) this dbf file with you geo-data based on the ZIP code
5) Symbolized your ZIP code layer based on the frequencies (count field from the Summarized data) - by ceating class renderer
I could not verify the details here, because I don't have ArcMap presently...
Maybe it finds no field from the same type as your other field (in the geo-layer).
You could verify what is the field type in your geo-layer, then verify if it is the same in your summarized field. If no, you could create a new field of this type and copy the values inside with the Calculate Field option.