There are two possibilities:
1. Are you intending to read the Excel file from the client's machine?
2. Are you intending to read the Excel file from the server side?
In Case 1. again two possible things:
a. Do you want to display the Excel file in applet, or,
b. Do you want to load the Microsoft Excel on the client machine to view the Excel file?
In Case 2. Do you want to display the Excel file in an applet?
Case 1a and Case 2:
If you want to display the excel file in an applet, first you need to develop a interpreter that will be able to interprete the Excel file and is able to display as it is. For that you need to look into the internals of the format of the excel file. All you have to do is that have an inbuilt interpreter into the applet. If you are accessing the file on the client's machine then you need to have the read permission granted to the applet. Check out the policy tool in JDK 1.2.
If you have the excel file on the server side then it is well and good. But in both the cases you need to implement the excel interpreter.
If you want to load the file from the client machine and use Microsoft Excel to view that file then your applet should have two permissions granted: Runtime permission to load the Microsoft Excel "exe" and Read permission to read the Excel file on the client's machine.