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First of all you need to install the sun java runtime, but ubuntu 10.04 don't have the necesary repository to install it.
Step 1> ADD REPOSITORY
Go to Console and run this > sudo add-apt-repository "deb http://archive.canonical.com/ lucid partner"
Update the repository > sudo apt-get update
Step 2> INSTALL SUN JAVA RUNTIME
Install the package > sudo apt-get install sun-java6-jre
You need to accept the terms and conditions to install it.
Step 3> DOWNLOAD PLATINGUI FOR LINUX
Use this link > DOWNLOAD
Step 4> INSTALL PLATINGUI USING JAVA RUNTIME
Find your download in your Console i.e. cd /home/yourusername/Downloads/
Run the command > sudo java -jar PlatinGUI-Linux-7.20rev2.jar
A menu will appear > Press Next > the path will be /opt/SAPClients > and that's all.
In your toolbar will appear the submenu and icon of your SAPGUI.
Step 5> CREATE YOUR CONNECTIONS TO YOUR SERVERS
In the menu File > New Connection > Put a name on Description > Go to Advanced Tab > tag Expert Mode > and write a line like this > conn=/H/SAPServerIP/S/Port > i.e. conn=/H/192.168.1.50/S/3200
If you don't know the port, the port is 32 and the number of the system, 00, 01, etc.
I wish that it will help you !