You need to slide the card in until it is firmly seated. If the card is moved to the other of the two slots, you may need to change related settings in the network software which identify the network hardware to use. Almost all Macs to date have a SCSI port. This is a kind of fast multi-purpose external expansion bus. It is used often for devices such as hard disks and scanners.
Ethernet adapters are clunky and expensive, but nonetheless a useful last resort.
ID 7 will be the Macintosh computer itself. SCSI ID 5 is flaky on some PCI PowerMacs and should be avoided. If you have multiple external SCSI devices, watch out for problems and test each device with the computer separately before trying to get things working together. Cheap cables are often found to be the sort of annoying otherwise untraceable errors. Over-termination or lack of termination will mess it up.