The riding scene in Kamloops is actually mostly under-the-radar because a lot of trails are illegal (on private land) so you may not get much information from the bike shops
Some authorized trails are:
Kenna Cartwright park (spiders web of short xc trails on the edge of town);
Kamloops Bike Ranch (big hits, jump park, limited xc) but excellent facility considering maintained by local council. There's also a shuttle ($2 per uplift and well worth it). The KBR is in town;
Harper mountain trails are more freeride and you'll want a shuttle vehicle and is quite a way out of town (actually, I'm not sure of the legality of these trails);
Sun Peaks bike park but may be shut by the time you arrive - google them. It's not anywhere as big as Whistler but it's lot quieter. It's about 45 mins drive from town.
Dew Drop trail is well worth the effort (can be shuttled if you have two cars) but it's perfectly doable on an xc bike. That's out by the airport.
There's also some xc stuff out on the Lac Du Bois area north of town.
Now, the goodies...
The info for legal stuff is just a google-away but when you drive along the highway, you'll see lovely singletrack trails EVERYWHERE!
E-mail Ted Morton: email@example.com and tell him I sent you. I used Ted as a guide/shuttle earlier this year and was blown away by his service and very reasonable pricing. He's involved in the local race scene and also teaches mountain biking at some of the local schools and knows the local access issues
(Seriously, e-mail Ted!)