On 64 bit Windows boxes, programs like QuickBooks that are still 32 bit programs will be stored in C:\Program Files (x86)\ . All programs on your 32 bit computers, will be stored in C:\Program Files\. Since you need to specify the path to the external app you want to launch, the external program link will be broken on your 64 bit computers if you specify a 32 bit path, or vice versa for your 32 bit computers if you specify the 64 bit path.
So let's say I want to create a QuickBooks ToolBar button. Here are the steps:
1) I currently have more 32 bit PCs than 64 bit PCs, so I'll use the 32 bit path in OD like so:

(I select "All modules" under "Add a button to these toolbars" at the middle left because it seems more convenient to have the button in the uppermost OD toolbar). Click "OK" to save the Program Link.
2) Next, on the 64 bit PCs, find the folder that the program is in (in my case C:\Program Files (x86)\Intuit). Right click on this folder and select "Create Shortcut". Windows will tell you it can't create the shortcut in the folder you're in, and will ask you if you want it on the desktop. Click "Yes"
3) Rename the folder from "Foldername-Shorcut" to "FolderName". So in my case it would be "Intuit-Shortcut" to "Intuit"
4) Open up C:\Program Files in Windows Explorer. Then, right click on the shortcut you just created on your desktop, and drag it there. A context menu will popup. Select "Move here" from the list of choices.
5) Repeat this process on all of your other 64 bit computers.
Now the OD toolbar button for your external program will launch your program from any PC that you have it installed on, regardless of whether it is 32 or 64 bit.