Anything else beside WinISD

That bit of geek-speak in the dialogue box is the way of telling you that the program is trying to make a calculation and a number doesn't add up.  It could happen if you type in a new model name and then forget to go to the 'parameters' tab to input the numbers.  It can also show up if you don't input enough parameters in that field....and an error can also occur if you input all the numbers but they don't add up to what the program figures...

Such as 'Qts', which is derived from the values 'Qes' and 'Qem'.  If you put in both 'Qes' and 'Qem' in the parameters field, then you should just Tab (click) through 'Qts' and you'll see the calculated number fill in using a different color.

When you see that "Floating point..." box, go back to the parameters tab of the woofer you're trying to work with.  You pull it up by using the top tool bar drop-down menu > Utilities > Database Editing > find your speaker from the scroll list and go to the parameters tab.  Type in the T/S parameters.

Play around with the program for awhile and you'll see how things work.  It would be worth the effort to be able to use a free program...and there's no spyware embedded either.  I close the program out and then bring it back up whenever that  "Floating...." box pops up.  Then I know to go to the parameters field and see if I've put in some T/S values for that particular woofer.

Sure, there are many enclosure modeling programs out there.  Run a google search and you'll quickly come up with many.  Shoot, there's even a free one called BlauBox on this site under downloads->software.

Some are very expensive, complex, and full featured.  Others, like "Sealed Designer," are very very simple. 

I personally always recommend WinISD because it is 1) free 2) relatively easy to use and already has a huge driver database 3) relatively powerful for quick guestimates

For practical advice on using it, see Stevdart's helpful post above.