Basically, follow the instructions in the Ubuntu site https://help.ubuntu.com/community/CitrixICAClientHowTo#Citrix_ICA_Client_12.1_on_Ubuntu_11.10_64-bit
They have a link to a patched version of the broken official package. Alternatively, download the official one and fix as follows:
Extract control.tar.gz:
$ mkdir fiddle
$ cd fiddle
$ ar x ../icaclient_12.1.0_amd64.deb control.tar.gz
Untar it
$ mkdir control
$ cd control
$ tar xzf ../control.tar.gz
Fix postint as instructed in the Ubuntu community page, ie, replace 'echo $Arch|grep "i[0-9]86" >/dev/null' with 'echo $Arch|grep "x86_64" >/dev/null' at line 2648
Set permissions to root:root to avoid being told off by Ubuntu
$ sudo chown root:root *
Create a new control.tar.gz
$ rm ../control.tar.gz
$ tar czf ../control.tar.gz *
Create a deb with the new control.tar.gz replacing the old one
$ cp -i ../icaclient_12.1.0_amd64.deb icaclient_12.1.0_amd64_patched_postinst.deb
$ ar r icaclient_12.1.0_amd64_patched_postinst.deb control.tar.gz
Now you can install this normally (ie once you've installed all the depended packages)
You will probably also need to add the certificates to Citrix, just copy them from Firefox:
$ sudo cp -i /usr/share/ca-certificates/mozilla/* /opt/Citrix/ICAClient/keystore/cacerts