Tom,
The Electron uses Acorn's Advanced Disc Filing System (ADFS), as opposed to the DFS which is more common with the BBC Micro. Single-sided DFS disc image files are usually .ssd files and this is what BBC Explorer produces. You need a program like Jon Welch's
ADFS Explorer to create an ADFS disc image file (.adf). You should be able to drag and drop your image files into a .adf disc image which can then be loaded into ElectrEm.
Once you've got access to the file in ElectrEm (use *CAT to catalogue the disc image from the ADFS filing system), I assume you need to *LOAD the image from the correct MODE. You may need to specify the correct load address, if it isn't set correctly - you'll probably get more help about Electron / ElectrEm-specific questions on the
ElectrEm Yahoo! Group.
Sam.