ID doesn't have a button for it like Quark or Word, but you can do the same thing. Select the text and angle it. Ten to fifteen degrees is usually about right.
If you have a lot of text to do this to, add the angle to a style.
To expand on Peter's responses ...
You can oblique text as he describes, however, that does not make a true italic style. It is only a “slanted” version of the orignal style. A number of san serif font families have oblique styles instead of true italic styles where such slanted (“obliqued”) version of typefaces are provided as part of the font family although in many such cases, the font style is still labelled (or maybe better put, mislabelled) as “italic” as opposed to “oblique.” This hackery is much less common for serif font families.
A true italic font actually have a different design that the underlying “normal” / “regular” style. Typically, these differences are most pronounced in a lower case “a” or “g” for example.
You may want to point this out to your client, contrasting text rendered with the artifically obliqued font with that rendered with a true italic font in a similar font family. They may or may not actually care, but at least they won't complain about it afterwards.