When I add text to a sign it displays for a second then disapears. 

I think it might default to white text on a white background. It can also happen if the text is too large for the sign (too large of a font or too many characters). Copy and paste this to use an example of black text on a white background:

/silt "Lorem Ipsum" 16 #000000 #FFFFFF

Merged post

It fits on even a small sign, btw (16 point font).
If you want to use different colors: 

https://www.color-hex.com/

Thanks, but that didn't seem to help.

I put a AirWolf vender in OutPost and I'm trying to put a sign pointing to him.

I added a huge wood sign using MonumentAddons.

when I use the /silt command with just the text I get a message saying it was added to the cue then a message that it was added to the sign.
checking the log it shows also that it was added to the sign, But nothing shows.
I tried it again copy/pasting the command as you gave it. Same thing, Says it was added but nothing shows.
I tried using the link to the download from the logfile with the /sil command. Says qued then added but no show.

I added a small wooden sign, same thing.

I deleted the plugin, the config, the logfile and readded it. still the same

If I open the sign and scribble on it with the games editor and save I see it for a second after exiting the editor.

I know it worked for me in the past, it was only a couple of months ago I used it to label the floors on my farm