There are several version of the OSCAT libraries available. Could you please tell us which one you are trying to import?
The older Version of this library is based on 32 bit and ctrlX CORE is based on 64 bit so it would not fit. See also topic "Is there an OSCAT Network library supported in ctrlX Core?".
See also some information about installing the package in topic "Smart Function Kit using a ctrlX CORE connected through EtherCAT".