Why won't you legacy extension install?

From Joomla! Documentation

lthough Joomla! 1.5 allows backward compatability with 1.0 extensions, there are some times when a 1.0 extension does not install or work as intended.

Below are some suggestions to make them work.

• Make sure you have enabled the legacy mode in your plugin manager.
• Joomla! 1.5 requires strict XML whereas Joomla! 1.0.x did not. Look at the XML file (there is one in every extension) and see if there are any problems. The most common problem seems to be the character & which needs to be replaced with & a m p; (remove the spaces!). Any other special characters also need to be replaced in this way. There are numerous sources on the web that can show you how to translate special characters.
• Read your error messages. :) Although some are complicated, many involve simple things. For example in many places things that used to be named beginning with the string "mos" now have new names (usually the same, but with the mos deleted). If your error messages say mosSomething not found go edit the file and take off the mos.
• Related to that, if your error message says file not found, see if the file has been moved to a new directory.
• If these do not work try going to the homepage for the extension (these can be found in the listings in JED). Many people are trying to do the same thing you are, so you will probably get some useful advice.

If you do get an extension work by making any of these or other changes, the nice thing to do is to send a message to the extension's developer so that the patches can be available to others.