I don't mean to be rude here, but the concept of going through Access to get from Excel to Powerpoint strikes me as a stupendously bad idea.
Also, I do a lot of office automation and I've never heard of this DDEInitiate thing you're using, so I suspect it's an older form of office automation, or maybe it's something you have to use in VBA (I do most of my stuff from VB, not VBA).
I'd suggest that you use a VB app to go from Excel to Powerpoint rather than embedded VBA in Excel. Include the Excel and Powerpoint object libraries in a VB project and then use the object browser to find the methods you want. I believe you can then use the methods in VBA if you really need to stay in Excel.
I agree it sounds mad but I haven't got access to VB itself at work and probably couldn't get it if I asked so I've only got access to VB through Excel, Access etc.
I found that using the DDE tools I could start up Powerpoint iteself but then not be able to use Sendkeys on it and couldn't open a specific file, or I could open a specific Access file - very annoying!
My boss has a spread sheet with loads of various company project's data on it and he wants to be able to click on a button next to a set of data and it produce a powerpoint slide/presentation with various stats in it in a presentable format in it. I've produced a sheet that looks the way he would like the powerpoint slide to look that's linked to all the appropriate data so it's basically a case of opening powerpoint and importing this one sheet into it.