Is it possible to get call waiting on Travel SIM?

If so, how?

Not sure about Travel sim. But, almost all the sims I have (All the international ones, riiing, globaslsim and yackie, for sure), came with call waiting disabled. I just enable it via the phone's menu. That's it! So give that a try.

Tried menu on phone and *43# both come up with "Password Error" for some strange reason

AFAIK, all phones have a call waiting menu where you can activate, deactivate, check status of call waiting for all types of calls, voice, data, fax(depending on phone).

Yes, you're right. Phones have menus for managing call waiting as well as call diverting/forwarding, barring, enabling/disablling caller ID. There are also underlying GSM network codes composed of */#/digits (e.g. *43# for call waitng activation as Keighleyboy mentioned) which do the same.
But in the real world there are also the following possbilities for managing the services in question:
1) Non-standard operator-dependent codes (USSD)
2) Voice prompts + DTMF
3) Web-accessible user accounts
4) Call/e-mail to Customer Support

It's also possible that it's the operator policy not to let users enable/disable services they need at all :P .

I used TravelSim in Malta but I forgot to try... well, I'll do it in Italy now and I'll inform you later

Thanks for the help

well, sorry for the delay. I tried to have a waiting call here in Italy without any success :(