Am I right in thinking you're trying to sync to a track that's already recorded?
If so, try this. (It’s a bit long winded but it’s a pretty accurate method and the one I prefer to use).
1) Under File/Project settings, make sure the 'Timebase for events...' is set to 'Time'.
2) Under Options take the tick out of the box called 'Enable Snapping'
3) Play the track and hit the M key on the 1st beat of a bar and the 1st beat of the next bar. This will place two markers.
4) Expand the track so you can see the waveform between the markers clearly. If you want more accuracy, drag the markers slightly to precisely align them with peaks in the waveform that correspond to the beginning of the bar. Of course this will only work if have a sound on the 1st beat of each bar.
5) Left click drag (not on the waveform) to make a time selection that spans exactly between the two markers, as close as you can get it.
6) Right click and select one of the of the following options
(a) Set project tempo from time selection...
or (b) Create measure from time selection (detect tempo).
See picture below:-
Option (a) will set the global tempo for the whole song
Option (b) will allow you to set a tempo for each bar. If the tempo is varying slightly you could repeat this process throughout the whole song if you've got enough patience. Although there's a more elegant method of doing this as explained at a link in my earlier post.
The example is shemp's Snake_Charmer track, which I worked out to be 90 bpm using this method, so he can tell me if I got it right (or otherwise).
Pete