It must have told you to visit Tamatown. Enter Tamatown as usual and go to the travel agency and they'll give you the souvenir to take back with you.
I'll explain that again step by step:
1. On your Tamagotchi v4 or v4.5, go to the fourth icon (activities).
2. Select "Item."
3. Select the travel ticket.
4. Your Tamagotchi will depart on a journey, press any button to call it back.
5. You will be told to visit Tamatown.
6. Go to the fifth icon (communication).
7. Select "PC."
8. You will be given a password for Tamatown.
9. On Tamatown, enter in your username and password.
10. Go to the travel agency on Tamatown.
11. You will be given the souvenir.
12. Select "Logout" on Tamatown and you will be given a password.
13. On your Tamagotchi, press the B button.
14. Enter in your Logout password on your Tamagotchi.
15. On Tamatown, confirm that you have entered in your password correctly and you will be given your item password(s).
16. Go to the fourth icon on your Tamagotchi (activities).
17. Select "Password."
18. Enter in the password and you will get your souvenir. To view it, choose "Souvenir" on the activities menu.
