Did you look at the example that comes with the library?
The example from the Seeed Can library is a bit easier to understand:
You have to set the masks (there are 2) and then use the filters to only get the IDs that you want to receive
void setup() {
...
//set mask, set both the mask to 0x3ff
CAN.init_Mask(0, 0, 0x3ff);
CAN.init_Mask(1, 0, 0x3ff);
/*
set filter, we can receive id from 0x04 and 0x05, there are 6 filters
*/
CAN.init_Filt(0, 0, 0x04);
CAN.init_Filt(1, 0, 0x05);
...
}