How do I convert from an integer to a string?

Use to_string() (running example here):

let x: u32 = 10;
let s: String = x.to_string();
println!("{}", s);

You’re right; to_str() was renamed to to_string() before Rust 1.0 was released for consistency because an allocated string is now called String.

If you need to pass a string slice somewhere, you need to obtain a &str reference from String. This can be done using & and a deref coercion:

let ss: &str = &s;   // specifying type is necessary for deref coercion to fire
let ss = &s[..];     // alternatively, use slicing syntax

The tutorial you linked to seems to be obsolete. If you’re interested in strings in Rust, you can look through the strings chapter of The Rust Programming Language.