First, you need to find the GUID for the career. If the Sim still has the career, then the career GUID is in the Career tab for the Sim Description. Copy the GUID to the clipboard using <ctrl>C.

Otherwise, you can find it in SimPE by selecting the Tools menu, then PJSE, then Simantics Resource Finder. Check OBJD Name and then type "JobData - " (without the quotes) into the Name field. Click on Search and wait while it finds all of the job data. Look through the list until you've found the career that you want and copy the GUID to the clipboard using <ctrl>C.

Once you have the GUID that you want, edit the memory and select the Raw Data tab. The GUID goes into Raw Data 5 and 6, with the first 4 digits in data 6 and the last 4 digits in data 5. You can paste the GUID from the clipboard using <ctrl>V, then remove the unneeded digits. Commit and save.

[Update:]
You can include your CC in the SimPE Simantics Resource Finder search by going to Extra -> Preferences -> System Folders and ensuring that Savegame points to your save game, then going to Extra -> Preferences -> FileTable and clicking on "Add downloads folder". Your downloads folder will be added under Advanced Settings; make sure that the line "Recursive folder: (SaveGameFolder)Downloads\" is checked.