Sixteenth Notes

Straight up 16ths?  You can't.  Four 16th notes to a beat. That's the math of music.
 
However, you can set your resolution to 32nd notes and insert three 16ths and two 32nds.
Or....
You can also use triplets  and depending of course on the rhythm you are seeking.... you can place the 32nd notes in up to 8 places per beat.... or you can use 64ths and you have 16 possible places.
 
Depending on the tempo, this may not appear to the listener as short 32nds or shorter  64ths and it sounds like it's intended.
 
HOWEVER....
This may be a better alternative.  If you can play a keyboard, this is probably the absolute best way to get this done.  If you can actually play that section of music on a midi keyboard and use the click track to stay on time..... the software will interpret exactly what you played and put the appropriate notes exactly where you played them and assign the proper value to that note so that the playback will sound exactly like it should.
 
 

Then you will need to follow Guitarhacker's suggestions to perform the music or change the time signature.

chichow
I know there is 4 16th notes to a beat.  The music I'm trying to put into MC5 has 5 16th notes on beats 2, 3 & 4.

Like I said, you need to change the resolution to a higher resolution and then you can get a ton of notes into that same space. The higher the resolution, the more "time slots" become available to you for placing notes in a given space in the song.

I forget the actual numbers but if you set the resolution to it's highest setting, I think you can divide a single quarter note space into 240  notes/beats in the space of one quarter note in 4/4 time.  That gives you more than enough to place 5 notes in the space you want.
 
Like STXX pointed out, they will not be 16th notes at that point. BUT.... no one will know except you.