Problem. What is x3g file? CTC printer uses it from SD card. Can’t print STL or others 
That’s the sliced file that tells the printer how to move. Some printers use gcode, some use x3g. You need to slice the stl files before printing them.
x3g is makerbot’s new file format, which supports Sailfish. I have seen some that use replicatorG for it http://replicat.org/
What printer are you using?
@Adam_Steinmark
A ctc printer maybe?! 
@Jari_Laakkonen
You can’t print stl files directly. You need to slice them. Like adam already mentioned.
Okay. normally I print using USB and MakerBot software, but I think USB is not fast enough as it should be in this old computer. And new MakerBot 3.7 doesn’t run at all on it and I must use older version. And x3g file format doesn’t include to it. Or I just dont know how to convert to it.
X3g is actually not the newest file format for makerbot. You might be accidentally converting to the newest format instead of x3g.
I doubt USB is too slow even from an old computer, if it was you wouldn’t be able to run Makerware and it would crash. USB should be fine unless you’re noticing defects like skipping a line of code causing layers to offset. Otherwise youo could export your sliced model from Makerware to an SD card and print from there. And you should try to use a better slicer like Cura, Slic3r, KISSlicer, or MatterControl.
Thank you all guys. I’ll try something other slicer, hope they work better. But I think, I’ll actually need an new computer.
@Jari_Laakkonen what makes you think you need a new computer?
Almost 8 year old device with win8.1 (witch is noghtmare on this machine) Usb is 1 or 2 category and card reader do not regonize newer sdhd card. This is good and realible device, very good friend, but it is just too old, I’m afraid.
Just a heads up, if you have a makerbot clone, then you’ll probably be unable to use another slicer because it’ll export gcode and not x3g.
@Justin_Nesselrotte I know in Slic3r you can set your firmware to Sailfish to export x3g and I think Cura might have the option to export x3g as well. Haven’t had the need to try though.
Cura at least still requires makerware. I think Slic3r’s output for makerbot is just the gcode style before s3g/x3g came out
Well like I said I haven’t tried it.
You can convert gcode to x3g using GPX: https://www.thingiverse.com/thing:81425
BTW, controlling any 3D printer, at least the ones that don’t have an ARM processor, over USB will always be much slower than actual USB speeds because the USB device you connect to is a USB>serial converter, which has a maximum speed of about 250000 baud. If you’re printing with a lot of short segments, this is slow enough to cause the printer’s buffer to run dry, causing slowdowns or brief pauses in the print. Because of nozzle pressure lag, this will result in nasty print artifacts.
All Makerbot clones (CTCs, FlashForges, Wanhaos, etc) as well as genuine Replicator 1,2,2x printers use x3g. It’s a pre-processed command format that allows your computer to do some of the motion math for better printer performance. Makerware will automatically convert gcode to x3g when you slice the STL, as will ReplicatorG and Simplify3D. If you want to use other slicers like Slic3r or Cura, you have to set up the GPX plugin that @Whosa_whatsis linked as a post-processing script to do the conversion.
When you set Slic3r or Cura to “Makerbot gcode” that is telling it to output the gcode flavor that GPX expects. You still have to run GPX to get the x3g file.
The new 5th Gen Makerbots use a new .json based command format. It’s just a different way of structuring the same motion commands as you’d find in gcode. I believe Simplify3D is the only 3rd-party slicer that can export it right now.