I haven't used a USB printer before but perhaps I can help to provide some hints on the printcap though.
The spool directory has to exist: /var/spool/lpd/lexmark
The input filter program has to exist: /usr/share/printconf/util/mf_wrapper
The device has to exist and functional: /dev/usb/lp0
The accounting filter needs to exist or be removed: af=/var/spool/lpd/lexmark/lexmark.acct
Any programs that the input filter requires need to be installed. Like Ghostscript, etc. And since it uses the name mf_wrapper it could be a file generated by RedHat configuration tool or something. But this filter is responsible for converting the output properly for your Lexmark printer.
You might could try the following command and see if you get anything from the printer.
cat /etc/printcap > /dev/usb/lp0
If it does print it, you will probably see the staircase effect on each line. But that is OK because at least you know the device lp0 is working correctly. If you get nothing then you should at least check to see if your modules are loaded to support the USB printing. But without a proper filter the printer may not be able to print even a simple text file.
More detailed information can be found in the /usr/doc/Linux-HOWTOs/Printing-HOWTO and there is also an excellent web site available: http://www.linuxprinting.org.
Both of these may be able to provide detail information for setting up your Lexmark and/or best recommendation for support.
Hope it has helped.