You have been given an IP address and want to find the port which the machine that owns that ip address is plugged into.
First you need to find the mac address of the device
show arp | match 10.1.5.34
admin@ST3> show arp | match 10.1.5.34
00:00:09:fa:aa:3e 10.1.5.34 server1.domain.com vlan.1 none
Now you can find the interface
show ethernet-switching-table | match 00:00:09:fa:aa:3e
admin@ST3> show ethernet-switching table | match 00:00:09:fa:aa:3e
DOM 00:01:03:4a:15:3d Learn 0 ge-7/0/28.0
This tells you that the IP address 10.1.5.4 is on a machine which is plugged into port 28 on switch 7 of Stack3
