You should locate the file you want to use, in the same folder as the FLAC project file. Otherwise, the file may be called by specifying its complete path (e.g., g:\myfolder\gilroy1.acc)

As Hossein explained, you should place the input motion file in the same folder with the project file. By the way, you should use proper command to employ the motion. For example:

table 1 read ImperialValley.txt

apply xacc 0.01 hist table 1 range z 0.0

Also, in the first line of your input motion file you should define number of data and a coefficient like:

1000  0.01

Thanks for your helps

Hossein and Mohsen

Of course, 

It is better to change your data to shear stress and then import to flac as table.

Like this:

table 1 0 325000

table 1 0.05 235000

...

Do you know how do we create a new .acc file?

See my below video:

https://www.dalfak.com/w/mjea4m/فرمت-های-فراخوانی-شتاب-زلزله-برای-تحلیل-دینامیکی-نرم-افزار-FLAC-2D-فلک-دوبعدی

To create a ".acc" file you have to save ur file as ".acc".