How do you open folders by 'directory id' ?

Sorry about that.
If “1234” is a folder then its path ends with a colon:

tell app "Finder"
activate -- not necessary; brings "Finder" to the front
open folder "Macintosh HD:1234:"
end tell

Also, if 1234 is the real id of the folder then you might try this:

tell application "Finder"
open folder id 1234 of startup disk
end tell

The problem though is that “1234” is not the name of the folder, it is the “directory id” of the folder. As in it is the HFS ‘directory id’ of that folder.
The path to the folder may be: Macintosh HD:Documents:Eudora Folder:Attachments Folder:
But the ‘x-eudora-setting:45’ returns a string in the format: Macintosh HD:1234 Where ‘1234’ is the ‘directory id’ of the folder, not the name/path of the folder. (1234 is a number chosen as a representation. I believe that all folders have a 4 digit ‘directory id’. This relates to HFS having 2 levels of FAT table.)
I think that the begining on this line got interpreted as html. So here it is again: x-eudora-setting:45 Volume and directory id of the folder to which attachments are saved.
Use it like:

tell application "Eudora"
set my_var to setting 45
display dialog my_var end tell

This will display a dialog box with text in the format “HD_name:directory_id”, where HD_name is the name of the hard drive and directory_id is the 4 digit directory id number of the folder.

How about using this instead?

tell application "Eudora" 
set inboxFile to (file of mailbox "In") 
end tell 

set {od, AppleScript's text item delimiters} to {AppleScript's text item delimiters, ":"} 
set attachmentsFolderPath to ((text items 1 through -3 of (inboxFile as string)) ¬ 
as string) & ":Attachments Folder:" 
set AppleScript's text item delimiters to od 


tell application "Finder" 
open folder attachmentsFolderPath 
end tel