Adding Java support to internet browsers is obvious. At least it is for MSIE, Netscape and Mozilla. Since all you have to do is download J2SE Runtime from www.java.com and install it. During installation, it detects your browser's folder and automatically install the java plugins.
Unforunately, Sun "forgot" to add SeaMonkey to the detection feature.
So you have to add the plugin yourself.
After having installed the J2SE Runtime, go to the program's "bin" subdirectory (default is C:\Program Files\Java\jre1.5.0_11\bin\ for J2SE RE 1.5.0 b11) and copy all NP*.dll files to your SeaMonkey plugin's directory (default: C:\Program Files\mozilla.org\SeaMonkey\plugins\). Restart your browser and you got Java support! You'll have to repeat this operation each time you update your Java runtime.
For v1.5.0 b11, NP*.dll files are: NPJava11.dll, NPJava12.dll, NPJava13.dll, NPJava14.dll, NPJava32.dll, NPJPI150_11.dll and NPOJI610.dll.