What documents directory do you mean? Take this scenario and go from there.

1) Plug your N900 into your usb port and select mass storage.
2) You will get a "Nokia N900" drive. Copy your deb file here, e.g. "file.deb".
3) Unmount your phone.
4) Open a terminal
5) Type "root"
- Your terminal name will change to "Nokia-N900-42-11:~#"
6) Type: "cd /home/user/MyDocs"
7) Type "ls"
- You will see the contents of this directory, including "file.deb" which you copied.
8) Type "dpkg -i file.deb"

Obviously do not include the quotations above, they include what you need to type. And note that directory names are CASE-sensitive, so write them exactly as shown above.