You can read the manual from Help menu in the main screen. The document is made under the terms of the GNU Free Documentation License. Actually, the manual was created for Dia 0.5, but you still can use it for Dia 0.96.1.
taufanlubis@toshiba:~$ sudo apt-get install dia
Reading package lists… Done
Building dependency tree
Reading state information… Done
The following extra packages will be installed:
The following NEW packages will be installed:
dia dia-common dia-libs
0 upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 3301kB of archives.
After unpacking 21.4MB of additional disk space will be used.
Do you want to continue [Y/n]? y
WARNING: The following packages cannot be authenticated!
dia-common dia-libs dia
Install these packages without verification [y/N]? y
Get:1 http://archive.ubuntu.com gutsy/main dia-common 0.96.1-3 [2392kB]
Get:2 http://archive.ubuntu.com gutsy/main dia-libs 0.96.1-3 [716kB]
Get:3 http://archive.ubuntu.com gutsy/universe dia 0.96.1-3 [192kB]
Fetched 3301kB in 1m3s (51.9kB/s)
Selecting previously deselected package dia-common.
(Reading database … 115213 files and directories currently installed.)
Unpacking dia-common (from …/dia-common_0.96.1-3_all.deb) …
Selecting previously deselected package dia-libs.
Unpacking dia-libs (from …/dia-libs_0.96.1-3_i386.deb) …
Selecting previously deselected package dia.
Unpacking dia (from …/archives/dia_0.96.1-3_i386.deb) …
Setting up dia-common (0.96.1-3) …
Setting up dia-libs (0.96.1-3) …
Setting up dia (0.96.1-3) …