This should be fixed in the v10.5 installation, but in the mean time the attached .Reg file will fix the association. Just download, double click the file and confirm that you do want the settings added to the registry.
Note that this file assumes you are running v10 and have it installed at "C:\Program Files\AEI Software\IRRICAD Pro 10\winicad.exe\". You can modify this path using Notepad if necessary, but please note and retain the double backslashes and the backslash-quote combinations, they are important.
However, the usual caveat applies whenever you are changing registry settings - if you have any doubt about your ability to do this successfully, please seek expert advice. Making the wrong change to the registry can cause a lot of problems on your system.
Dan