Thanks for the advice. In the end, I was able to get it working using the method that you described. However, the software still behaved strangely in that it didn't work initially. I thought that the length of the path to the file (which was on a network drive) might be causing the issue, so I tried copying it to the root directory of my hard drive. Then it worked. I tried it in a deeper directory on my local hard drive, and it didn't work.
I also had tried putting a delimited text file in the root directory of my hard drive and it didn't work, so exporting as the MDB definitely helped.
In conclusion, I wonder if Monarch has an issue supporting external lookup files stored on network drives, or in directories with spaces. Anyway, thanks again for your help.
Glad you got it working. Handling paths and filenames is something that Monarch would inherit from Windows, so I doubt that's the issue. But moving it to a local drive would avoid any possible timing or sharing issues that you had on the network drive.
A side effect of long network paths is that the File() function returns a string, and this can exceed the 254 character limit of a normal text field in Monarch, so do check and use RSplit() or memo fields as appropriate if you need this data.