I have been using the SLPR printer port with a JetDirect print server and HP DeskJet 990C on eCS for years and it works great.
First install the printer driver and assign it to any available port. Then right-click on the printer object just created, choose 'Properties', then 'Output Port', then 'Install New Port'. Click on the 'SLPR' icon, then 'Install'. The icon should now appear in the list on the 'Output Port' page and a properties dialog comes up. Put the IP address of the JetDirect in the 'LPD Server' entry. I put 'LPT1' in the 'LPD Printer' entry (this may vary depending on your particular JetDirect Unit). You could also try the 'High Performance' checkbox.
Now make sure the 'SLPR' port is hi-lited on the 'Output Port' page of your new printer object and you should be ready to try printing.
Another way to go is install CUPS. It has a JetDirect backend built in. I have recently switched to using this and it also works great - especially using the HPLIP drivers from Paul Smedley and Alex Taylor's PSPRINT Postscript driver. It is kinda tricky to install though - see
http://svn.netlabs.org/ecups if you want to give it a go. Well worth it IMHO.
Regards,
Dave McKenna