What version of OSX are you running? We've seen this problem on newer OSX versions. Fortunately, the developer has created an update to get around this. This code will return what version of SemanticImport is installed:
"Version" /. PacletInformation["SemanticImport"]
After you run that, try running this (when you're online):
PacletUpdate["SemanticImport"]
This should install a newer version of SemanticImport. Let us know if this doesn't work for you.