This topic is discussed in the manual here and in this .zip file method video and this FPC .instr export video.
GETTING FREE INSTRUMENTS
This thread in the FL Studio Mobile forum has 100's of free (.zip & .instr) format instruments.
Custom instrument .zip * .instr IMPORT capability was added from FL Studio Mobile version 1.1 onward
See this page in the manual.
Once you HAVE added instruments you can get the *.instr files from your device (for sharing) by selecting the My Instruments folder in iTunes and clicking 'Save to...' and send to a location on your computer. The My Instrument folder is auto-created on first install of an instrument (double tap a .zip or .instr file).
EXPORTING & MAKING INSTRUMENTS
.instr EXPORT capability was added to FPC and DirectWave to support FL Studio Mobile.
FPC .instr export (new FPC version available from here):
DirectWave .instr export (new DirectWave version available from here):