Custom $INSTDIR

Im writing an installer for a web application...It starts off with a prompt asking the user to enter what they want to call the application(this will be used when creating the virtual directories and such). After this page is called I set InstallDir "C:\Inetpub\wwwroot\$appName"($appName is the value entered by the user on the first page). But when the MUI_PAGE_DIRECTORY page is shown the default directory shown is just C:\Inetpub\wwwroot the $appName variable is not concatenated to the install directory. I am calling my custom page before the MUI_PAGE_DIRECTORY page so the variable is getting set. Why is default install directory not being set with the variable?

InstallDir is parsed before any code is run. It sets the value of $INSTDIR. You could set $INSTDIR on your own once $appName is set.

This is probably a really stpuid question but....How would I set it, is there a function call I can use to set it?

Use StrCpy to set it just like you set any other variable.