FSX is not a program, it's a function of XS++ which is used to upload files to your phone's FS. Using FSX with CID49 is not as easy as CID51/52 where you just click a button and everything begins to upload. Since so many people PM me, I might as well just tell you guys here

I'll be using W800i's firmware and a CID49 K750i as an example.
1) Download a compatible MAIN firmware for your K750i, one that matches your customization files eg. APAC MAIN must be paired with APAC_1 custom files.
2) Slide the MAIN you downloaded with GSlide so it becomes compatible with your CID49 as all W800i's MAIN are initially CID36 and will not work on your phone.
3) Apply any VKP patch you want during the sliding process.
4) Start the sliding and wait for the process to complete... Walla! You now have a ready MAIN

Next,
1) Paste all files you want to upload into the "files_to_upload" folder within XS++ folder, with the correct directory structure eg. camera driver belongs in "ifs/settings/camera/".
2) Start XS++, and connect your phone.
3) Click "Customize File System".
4) Click "Settings" under "Configuration".
5) Click "BreakXS" for "Wayin".
6) Click "FSX" under "Configuration".
7) Click "Start FSX".
8) XS++ will flash "Breakloader" and you might be asked to unplug and reconnect the phone during this process.
9) Once XS++ has access to the FS, files in the "files_to_upload" will be transfered to the phone. When done, you might be asked to to unplug and reconnect the phone.

Finally,
1) In XS++, click "Flash" under "Configuration" and flash the MAIN you prepared earlier. DO NOT flash the File System.
2) When XS++ says it's done, it's done

You can now start using your phone again And a note:
<----- REMOVE THE SIM CARD AND MSPD BEFORE FSX AND FLASHING! ----->
And
<----- FOLLOW XS++'s INSTRUCTIONS! ----->
There, I think you guys can see the lines clearly

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Added on September 13, 2007, 10:26 pmIf you just want to flash the MAIN and FS, then skip the whole section on FSX. MAIN and FS must be prepared with GSlide. And this header must be used in GSlide for firmwares (MAIN and FS) with Chinese support:
"header__DB2010_49_red_344blocks.bin"

After flashing the FS, your phone needs to be customised.