To read/write Nintendo Power SF Memory cartridges you need the Adafruit Clock Generator installed and have the configuration switches set like so:
You also need both the Flash and SFM module enabled in config.h.
Dumping single games
After selecting the "SF Memory Cassette" option found in the "Super Nintendo/SFC" sub-menu open the "Game Menu" and select the game you want to save to the SD card. Sometimes you might have to power-cycle the cart reader before it works. Also on occasion the NP cartridge gets so confused that you need to boot it up on a real SNES first before it will work in the cart reader again. You can also read/write the save game in this menu.
Reflashing the whole cartridge
To flash something new you need to go to the "Flash Menu" under "SF Memory Cassette". Here you should make a backup of your existing mapping.map and flash.bin files and then write new bin/map files to the cart. You can get the bin/map files either from another NP cart or you can create those files with the SF Memory Binary Maker: https://github.com/moldov/SF-Memory-Binary-Maker
SF Memory Binary Maker needs the first 512KB of your original 4MB NP flash dump in a file named menu.sfc that needs to be copied next its .exe. In SF Memory Binary Maker you got two options: either create a standalone map file from a single rom up to 4MB or you can add multiple roms and create a combined .bin and .map file. The sum of all the roms together can't exceed 3.5MB however since you still need 512KB for the menu. Roms need to be unheadered and have a valid checksum.
If you don't want to use the Binary Maker you can also just flash the hi/lorom map from here and then just directly flash one single rom (up to 4MB).
In any case you should end up with a 512B map and 4MB bin file, rename both so their filenames are very short and without any special characters and copy them to the SD card. Then write the new flash and mapping files.