Hard to say without seeing a file. Some JSON files are not structured to easily import into a rectangular table.
JMP has two tools that might help: the JSON wizard, and the multiple file import. If you can make the JSON wizard import the file correctly, you can use that source script in the MFI JSON tab.
What version of JMP are you using?
If you have an addin that does what you need, you can use the JSL via view->addins, select the addin, open the addin directory, open the jsl files you find.
use the JSON settings from the script the wizard makes in MFI
Craige