insert silence!!

Why not just slice the audio region in two and separate the two regions as far apart as you want your silence.

jord

Use the marque tool, select where you want the silence, hit delete.

If you want destructive editing>
Double click on the file you want
This opens it in the Sample Editor
Select the portion you want to silence
Control + Delete

By insert a silence do you mean:
1a. break the audio, have silence then pick up the audio where you left off, making 2 audio regions with a gap between
1b. as 1a but still have 1 audio region
2a. silence a part of the audio file that currently has sound, but not affect the original audio file
2b. as 2a but save the silence in the audio file

1 and 2 need totally different methods, whereas the a and b of each require slightly extra work.

all can easily be done.

From within the sampler editor, go to the small functions menu, 5th command down > Silence.

Here's what I'd do
1 .make 2 copies of the audio file so that you don't accidentally damage the original
2. shorten the 2 regions so that one ends at the right point in the file and the other begins in the correct place.
3. re-position them so that they each begin and end where you want them to
If you want to join the 2 pieces, then
3. lengthen the left hand audio region so that it is long enough to fill the silent space. This will cause unwanted audio to play. So go into the audio edit window and silence that part of the region
4. use the glue tool to connect the 2 pieces together.