If a SYLK file opens as ASCII Text, it probably lacks a proper header. This happens with files from Accountant, Inc. from SoftSync.
For ClarisWorks to recognize a file as SYLK, the first line of the file must start with:
ID;POriginatingProgram
Just open the SYLK file as a Word Processing file, insert "ID;POriginatingProgram" into the first line, replace OriginatingProgram with the name of the program that created the file, and save as Text.