These are the steps I used to install the Citrix ICA Client (also called "Citrix Receiver for Linux") on my freshly installed Ubuntu 11.04 box.
First of all, install the following package: sudo aptitude install libmotif4
Download the latest .tar.gz from Citrix's website.
Unpack the archive and run the installer: sudo ./setupwfc
Just use the defaults when responding to the prompts.
Try to run the executable from the command line: /usr/lib/ICAClient
There should be no error (because we installed libmotif in the first step)
When I tried to open a secured .ica files (behind https) I got this error:
"SSL Error 61: You have not chosen to trust "YYYY", the issuer of the server's security certificate."
In order to fix that, execute the following command: cp /usr/share/ca-certificates/mozilla/* /usr/lib/ICAClient/keystore/cacerts/