Twisst looks up the location you have entered in your Twitter bio. When you simply fill in your geographic location, you should be fine and Twisst will start alerting you to ISS passes at that location.
Twisst sends a warning through Twitter to users if it could not geocode their location. If you receive such a message, change the location in your Twitter bio here: https://twitter.com/settings/profile
If you are not sure what locations works for Twisst, have a look at these examples.
What absolutely works best, is a combination of city and country name, such as 'Sydney, Australia', or 'Latvija, Riga'.
Country names alone are a bit generic, but locations like 'India' and 'Belgium' also seem to work well.
Coordinates work too, even when prefixed. Both '51.84605562,5.86992801' and 'iPhone: 51.826385,5.910294' will work fine.
Here are a few examples of locations Twisst won't be able to send out alerts for.
'Houston (that's in texas)' (just the geographic terms please)
'Earth' (a bit too generic)
'Provincie Frie' (half-complete province name)
'Central PA or Coastal DE' (too cryptic when you're not from around)
Your location can't be resolved to one set of coordinates if you string together your usual whereabouts like this: 'US, UK, EU, Asia' or 'Groningen / Den Haag'. So choose where you want to be and stay there ;-) Alternatively, you could also change your location often - Twisst will regulary look for updated locations.
There is a chance of false positives: a set of coordinates Twisst thinks is good, but which is actually wrong. An example is 'Below sealevel', which got resolved to the coordinates for Sealevel, North Carolina, USA. One Twisst follower put 'Orbiting' in his Twitter profile and ended up at Orbiting Drive in a small town in Wisconsin :-)
Another one is 'The Jungle (Northern Virginia)' which was resolved to a neighborhood called 'The Jungle' in Florida, USA . Twisst will send alerts out to these users, but the alerts they get are only accurate if that user really is there.
Until Twisst came along, it didn't really matter what you had up in your Twitter bio. These are a few of the funny locations Twisst did not quite know what to make of.
A Different City
At the edge of reality
die kant op (Dutch for: 'that way')
Heaven, Hell, Or Houston
Lala Land, USA
my own world
Third Stone from the Sun Twitterverse
27000 light-years from Sgr A*