If you want to record MIDI you’ll need a MIDI keyboard connected to a MIDI interface on your computer. This can be provided by a sound card or dedicated MIDI interface such as a USB device. Once the hardware is hooked up, you need to go into menu Options->Preferences and click on the MIDI tab. This is where to designate your MIDI input and output devices. Once you’ve done this, click on the VST Instruments tab and make sure your VSTi’s are listed there with a checkmark next to them. If not, add the directory that they are in to the search list so ACID can find them.
Now you’re ready to use a softsynth. Select menu Insert->Soft Synth and then select one of your VSTi’s. This will bring up the softsynth properties dialog. The first button at the top right looks like a MIDI connection and if you hover your mouse over it, it will say “Enable Realtime MIDI Alt-F7”. Press it (or press Alt-F7) so that its selected and you should be able to play your MIDI keyboard and hear the softsynth.
To record, just press the record button and select MIDI as the option at the top. Modify the filename and location if you wish, and at the bottom select the correct Record device (this should be your MIDI input device) and the correct MIDI Thru device which should be your softsynth. Finally press Start and begin to play.
