Cannot place hydraulic items on new design, only on old designs?
Have you edited a database creating one which has a problem? (This will not apply if the old designs use the same database as the new designs).
It may be that for some reason IRRICAD isn't creating the internal database. Check to see if the file ~Des~Int.mdb is present in the temporary directory (usually "C:\Documents and Settings\(log on name)\Local Settings\Temp"). If it hasn't created a recent ~Des~Int.mdb, make a copy of the Skeleton.mdb and place the copy in the temp directory, renaming it to ~Des~Int.mdb.
Check that the Skeleton.mdb still exists in the Irricad directory, and that the temp directory still exists (or that the path hasn't been changed to a non-existent folder - e.g. are there other recent ~Irricad type files in the "C:\Documents and Settings\(log on name)\Local Settings\Temp" directory. In a DOS box type SET. It will list your TMP directory and TEMP directory. Look in the directories listed - is there a ~Des~Int.mdb file in either of these?)
If IRRICAD is failing to create the temporary internal database, re-install IRRICAD.