Rexml - please help

Try:

xml_file = File.join(RAILS_ROOT,'images/guitars.xml')
File.open(xml_file)

(There's no actual need for the temp variable, I just thought it
looked clearer)

'/images/guitars.xml' tells file open to look in '/images' (as in,
anchored to the root directory for the system) for guitars.xml.
File.join( ) makes a full path out of the arguments you give it.
RAILS_ROOT is a constant set by rails as the root directory of the
project.

Regards
jon