First, welcome to the community. I will admit, it has been some time since I've used JMP 11 since we are now at JMP 17. Can you post screen shots of the window you get when you use the "open" option and select the file you want to open? Also, perhaps, attach the excel file so we can take a look at possible formatting issues.
As an option, you can typically select all of the data and headers in Excel that you want to import, copy and then open a JMP table and "Paste with Column Names".
"All models are wrong, some are useful" G.E.P. Box