I'd be surprised if there was any problem as Remote Buddy uses the normal keyboard emulation interfaces of OS X, which works with pretty much all applications and can be used to send keyboard events to Plex in other contexts as well.

Please provide more information on your setup and the problem itself:

1) Which versions of Mac OS X, iPhone OS, Remote Buddy and Plex are you using? Are they up-to-date?

2) In which part of Plex do you want to use the AJAX Remote's keyboard feature? Spontaneously, I couldn't find any entry field that I could use for testing?

3) Have you tapped outside the entry field and hit the button below the text field to send the text you've typed?

Best regards, 
Felix Schwarz