In order to export a waveform from cadence fisrt you have to select the format in which you want this waveform to be saved. This is done by using the -f <format> option in the ADE->Setup->Environment usrcmdline. for format I used "psfascii" but you have other formats available (use spectre -h for more info). This format has a header folowed by the time and the saved variables value for that time so you can preprocess and than import it in matlab. You will find the save that in the simulation directory for you schematic in the tran.tran file (if u do a transient simulation).