anyone know how to turn it on?

On my SP5m, if you're writing a new SMS, email, etc. - you press and hold the * key and you can then choose whichever input method you want. Alternatively, you can just tap the * key and it will cycle through all the imput mode choices.

Runningtiger is corredt. In some cases, predicitive text is not on, but can be turned on by using the asterisk key to cycle between all of the text input options. There should be 6-7 different options, but T9 should be the one you're looking for (as opposed to Abc, ABC, abc, t9, and 123).