Check the DDE documentation in the SAS manuals. The DDE application data needs to be a triplet consisting of the word "excel", then name of the sheet, and the location of the data in the sheet. Here is an example form the SAS manual
filename test dde 'Excel|c:\excel\
If that doesn't work, it's time to call SAS Tech Support.
Excel for DDE requires two connections and Doc is quite correct in referring to the syntax needed for sending and retrieving data from a specific spreadsheet.
Unfortunately, what is also needed is an instance of Excel with a system connection so that file open / save and similar functions can be performed. It is this instance that is failing.
Within Enterprise Guide, there is a standard setting that blocks sending system commands or operating system calls to a server. As long as the system command block is in place, SAS cannot send the string to start Excel or anything else.
There are workarounds, and a call to Tech support, or a visit to the website will shed light.