You have to import as a volume or surface area (see the option button in the file selection window), if you choose "graphic", you won't be able to get anything out of it.
You have to select the type of file you want to open (STL or STEP...) because there you have all the files... Once you have selected the file type, the operations box will be available because you can select operations according to the type of file you are going to open.